drm/msm: Reorganize msm_gem_shrinker_scan()

So we don't have to duplicate the boilerplate for eviction.

This also lets us re-use the main scan loop for vmap shrinker.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210405174532.1441497-3-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>