Revert "drm/mgag200: Add a workaround for low-latency"

This reverts commit bfa4437f.

This workaround doesn't work reliably on all servers.
I'll replace it with an option to disable Write-Combine,
which has more impact on performance, but fix the latency
issue on all hardware.

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517151050.624797-2-jfalempe@redhat.com