Comment 9 for bug 1752739

Well at least here on nvidia-755m if nvidia_drm modeset is enabled gdm3 fails to boot.
However lightdm works fine & prime sync is enabled..

$ sudo cat /sys/module/nvidia_drm/parameters/modeset
[sudo] password for doug:
Y
doug@doug-Lenovo-IdeaPad-Y510P:~$ xrandr --verbose
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 16384 x 16384
eDP-1-1 connected primary 1920x1080+0+0 (0x46) normal (normal left inverted right x axis y axis) 345mm x 194mm
 Identifier: 0x42
 Timestamp: 11726
 Subpixel: unknown
 Gamma: 1.0:1.0:1.0
 Brightness: 1.0
 Clones:
 CRTC: 0
 CRTCs: 0 1 2
 Transform: 1.000000 0.000000 0.000000
             0.000000 1.000000 0.000000
             0.000000 0.000000 1.000000
            filter:
 EDID:
  00ffffffffffff0030e4d90200000000
  00150103802313780a15d59e59509826
  0e505400000001010101010101010101
  0101010101017e3680b070381f403020
  350059c2100000190000000000000000
  00000000000000000000000000fe004c
  4720446973706c61790a2020000000fe
  004c503135365746312d544c4232004b
 PRIME Synchronization: 1
  supported: 0, 1