Comment 18 for bug 1561902

Odd, in theory that change should only affect the MTRR cache mode of the video memory for the generic VESA framebuffer driver. And while that is used initially it gets replaced later on by the special driver for radeon.
Maybe two things: First, after completing boot into an affected kernel, could you do a "cat /proc/mtrr" and add the output of that to this bug report.
Second, looking at the documentation it looks like it should be possible to disable that caching mode for vesafb with a kernel command-line option. If you look at /etc/default/grub, there should be a GRUB_CMDLINE_LINUX_DEFAULT (ususally it has "quiet splash" there). If you add a "video=vesafb:mtrr:0" there. Then run "sudo update-grub" and reboot. After reboot this should show up in the output of "cat /proc/cmdline" and in theory should make any "write-combining" that may have been in "cat /proc/mtrr" before go away.
Just as an additional note, looking at the dmesg output in this report, there seems to be something odd in the way the BIOS initializes MTRR. This might be the reason this specific laptop suffers from the change.