I am getting the same with libdrm-2.4.74, mesa=13.0.2, crashes when the html5test web page is opened in the Konqueror browser with QTWebengine-5.7.1:
libdrm-2.4.74/nouveau/pushbuf.c:727: nouveau_pushbuf_data: Assertion „kref“failed

[New Thread 0x7fff655dc700 (LWP 21049)]
konqueror: /var/tmp/portage/x11-libs/libdrm-2.4.74/work/libdrm-2.4.74/nouveau/pushbuf.c:727: nouveau_pushbuf_data: Předpoklad „kref“ nesplněn.

Thread 1 "konqueror" received signal SIGABRT, Aborted.
0x00007ffff7767228 in raise () from /lib64/libc.so.6
(gdb) where
#0  0x00007ffff7767228 in raise () from /lib64/libc.so.6
#1  0x00007ffff77686aa in abort () from /lib64/libc.so.6
#2  0x00007ffff7760167 in ?? () from /lib64/libc.so.6
#3  0x00007ffff7760212 in __assert_fail () from /lib64/libc.so.6
#4  0x00007fffe0619e54 in nouveau_pushbuf_data (push=push@entry=0x6d0d80, bo=0x6a58b0, offset=331932, length=496)
    at /var/tmp/portage/x11-libs/libdrm-2.4.74/work/libdrm-2.4.74/nouveau/pushbuf.c:727
#5  0x00007fffe0619d9b in nouveau_pushbuf_data (push=push@entry=0x6d0d80, bo=bo@entry=0x0, offset=offset@entry=0, length=length@entry=0)
    at /var/tmp/portage/x11-libs/libdrm-2.4.74/work/libdrm-2.4.74/nouveau/pushbuf.c:719
#6  0x00007fffe0619ee9 in pushbuf_submit (push=push@entry=0x6d0d80, chan=<optimized out>, chan=<optimized out>)
    at /var/tmp/portage/x11-libs/libdrm-2.4.74/work/libdrm-2.4.74/nouveau/pushbuf.c:330
#7  0x00007fffe061a18f in pushbuf_flush (push=push@entry=0x6d0d80) at /var/tmp/portage/x11-libs/libdrm-2.4.74/work/libdrm-2.4.74/nouveau/pushbuf.c:405
#8  0x00007fffe061ad50 in nouveau_pushbuf_kick (push=0x6d0d80, chan=<optimized out>) at /var/tmp/portage/x11-libs/libdrm-2.4.74/work/libdrm-2.4.74/nouveau/pushbuf.c:779
#9  0x00007fffe0f54d06 in PUSH_KICK (push=<optimized out>) at /var/tmp/portage/media-libs/mesa-13.0.2/work/mesa-13.0.2/src/gallium/drivers/nouveau/nouveau_winsys.h:59
#10 nv50_flush (pipe=0x2733370, fence=<optimized out>, flags=<optimized out>)
    at /var/tmp/portage/media-libs/mesa-13.0.2/work/mesa-13.0.2/src/gallium/drivers/nouveau/nv50/nv50_context.c:40
#11 0x00007fffe0c2bc1b in st_finish (st=st@entry=0x276c570) at /var/tmp/portage/media-libs/mesa-13.0.2/work/mesa-13.0.2/src/mesa/state_tracker/st_cb_flush.c:98
#12 0x00007fffe0c2bc80 in st_glFinish (ctx=<optimized out>) at /var/tmp/portage/media-libs/mesa-13.0.2/work/mesa-13.0.2/src/mesa/state_tracker/st_cb_flush.c:136
#13 0x00007ffff39916a9 in QOpenGLWidgetPrivate::beginCompose (this=0x2667c50) at kernel/qopenglwidget.cpp:727
#14 0x00007ffff395f629 in QWidgetPrivate::sendComposeStatus (w=<optimized out>, end=end@entry=false) at kernel/qwidget.cpp:12227
#15 0x00007ffff395f5e7 in QWidgetPrivate::sendComposeStatus (w=<optimized out>, end=end@entry=false) at kernel/qwidget.cpp:12233
#16 0x00007ffff395f5e7 in QWidgetPrivate::sendComposeStatus (w=<optimized out>, end=end@entry=false) at kernel/qwidget.cpp:12233
#17 0x00007ffff395f5e7 in QWidgetPrivate::sendComposeStatus (w=<optimized out>, end=end@entry=false) at kernel/qwidget.cpp:12233
#18 0x00007ffff395f5e7 in QWidgetPrivate::sendComposeStatus (w=<optimized out>, end=end@entry=false) at kernel/qwidget.cpp:12233
#19 0x00007ffff395f5e7 in QWidgetPrivate::sendComposeStatus (w=<optimized out>, end=end@entry=false) at kernel/qwidget.cpp:12233
#20 0x00007ffff395f5e7 in QWidgetPrivate::sendComposeStatus (w=<optimized out>, end=end@entry=false) at kernel/qwidget.cpp:12233
#21 0x00007ffff3937709 in QWidgetBackingStore::qt_flush (widget=0x75a5d0, region=..., backingStore=<optimized out>, tlw=0x75a5d0, tlwOffset=..., 
    widgetTextures=<optimized out>, widgetBackingStore=0xb17e20) at kernel/qwidgetbackingstore.cpp:132
#22 0x00007ffff3938ed9 in QWidgetBackingStore::flush (this=this@entry=0xb17e20, widget=widget@entry=0x0) at kernel/qwidgetbackingstore.cpp:1431
#23 0x00007ffff393a442 in QWidgetBackingStore::doSync (this=this@entry=0xb17e20) at kernel/qwidgetbackingstore.cpp:1323
#24 0x00007ffff393aded in QWidgetBackingStore::sync (this=0xb17e20) at kernel/qwidgetbackingstore.cpp:1179
#25 0x00007ffff3959a9f in QWidgetPrivate::syncBackingStore (this=0x75c3f0) at kernel/qwidget.cpp:1962
#26 0x00007ffff3971c08 in QWidget::event (this=this@entry=0x75a5d0, event=event@entry=0x268a660) at kernel/qwidget.cpp:9100
#27 0x00007ffff3a6fb0b in QMainWindow::event (this=this@entry=0x75a5d0, event=event@entry=0x268a660) at widgets/qmainwindow.cpp:1543
#28 0x00007ffff592c9d7 in KMainWindow::event (this=this@entry=0x75a5d0, ev=ev@entry=0x268a660)
    at /var/tmp/portage/kde-frameworks/kxmlgui-5.29.0/work/kxmlgui-5.29.0/src/kmainwindow.cpp:867
#29 0x00007ffff5971295 in KXmlGuiWindow::event (this=this@entry=0x75a5d0, ev=ev@entry=0x268a660)
    at /var/tmp/portage/kde-frameworks/kxmlgui-5.29.0/work/kxmlgui-5.29.0/src/kxmlguiwindow.cpp:118
#30 0x00007ffff7b504e4 in KonqMainWindow::event (this=0x75a5d0, e=0x268a660) at /var/tmp/portage/kde-apps/konqueror-16.12.0/work/konqueror-16.12.0/src/konqmainwindow.cpp:5494
#31 0x00007ffff392aacc in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x75a5d0, e=0x268a660) at kernel/qapplication.cpp:3799
#32 0x00007ffff39324c6 in QApplication::notify (this=0x7fffffffd300, receiver=0x75a5d0, e=0x268a660) at kernel/qapplication.cpp:3556
#33 0x00007ffff2ea2b7a in QCoreApplication::notifyInternal2 (receiver=0x75a5d0, event=event@entry=0x268a660) at kernel/qcoreapplication.cpp:988
#34 0x00007ffff2ea51bb in QCoreApplication::sendEvent (event=0x268a660, receiver=<optimized out>) at kernel/qcoreapplication.h:231
#35 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0, data=0x61ab10) at kernel/qcoreapplication.cpp:1649
#36 0x00007ffff2ea5628 in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1503
#37 0x00007ffff2eef6d3 in postEventSourceDispatch (s=0x667670) at kernel/qeventdispatcher_glib.cpp:276
#38 0x00007fffed0995e7 in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#39 0x00007fffed099840 in ?? () from /usr/lib64/libglib-2.0.so.0
#40 0x00007fffed0998ec in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#41 0x00007ffff2eef74f in QEventDispatcherGlib::processEvents (this=0x670e90, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#42 0x00007ffff2ea1b3a in QEventLoop::exec (this=this@entry=0x7fffffffd1b0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:212
#43 0x00007ffff2ea940c in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1261
#44 0x00007ffff341b39c in QGuiApplication::exec () at kernel/qguiapplication.cpp:1633
#45 0x00007ffff392aa25 in QApplication::exec () at kernel/qapplication.cpp:2975
#46 0x00007ffff7b929c0 in kdemain (argc=1, argv=<optimized out>) at /var/tmp/portage/kde-apps/konqueror-16.12.0/work/konqueror-16.12.0/src/konqmain.cpp:233
#47 0x00007ffff7754720 in __libc_start_main () from /lib64/libc.so.6
#48 0x00000000004006e9 in _start ()