drm/msm: remove unnecessary mmap logic for cached BOs

No one knows what this is for anymore, so just remove it.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Acked-by: Jordan Crouse <jordan@cosmicpenguin.net>
Link: https://lore.kernel.org/r/20210423190833.25319-2-jonathan@marek.ca


Signed-off-by: Rob Clark <robdclark@chromium.org>