The current code should do the right thing with respect to disabling display
access to vram when we reconfigure the memory controller.  The current code
disables memory reads but leaves the display controllers enabled while we
change the MC setup.  Turning off the crtcs as the patch you mentioned does has
two problems:
1. it breaks some systems which the current method fixes
2. it defeats the purpose of GRUB_GFXPAYLOAD_LINUX=keep which is to avoid
turning off the displays for flickerless boot up.  If you turn off the crtcs
you have to re-init the entire display pipeline.
The problem seems to be that disabling the crtc memory reads seems to take
longer than expected on some systems which leads to invalid reads while the MC
is being reprogrammed.  One possible solution may be to leave the MC as
configured by the vbios and try and put the gart aperture either before or
after the location of varm in the GPU's address space.