drm/msm: Fix a few comments

Fix a couple comments which had become (partially) obsolete or incorrect
with the gpuvm conversion.

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/667237/