drm/msm: Small msm_gem_purge() fix

Shoot down any mmap's *first* before put_pages().  Also add a WARN_ON
that the object is locked (to make it clear that this doesn't race with
msm_gem_fault()) and remove a redundant WARN_ON (since is_purgable()
already covers that case).

Fixes: 68209390

 ("drm/msm: shrinker support")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210405174532.1441497-8-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>