drm/msm: Drop mm_lock in scan loop

drm/msm: Drop mm_lock in scan loop

lock_stat + mmm_donut[1] say that this reduces contention on mm_lock
significantly (~350x lower waittime-max, and ~100x lower waittime-avg)

[1] https://chromium.googlesource.com/chromiumos/platform/microbenchmarks/+/refs/heads/main/mmm_donut.py

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20210402211226.875726-1-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>