drm/msm/gem: Move locking in shrinker path

Move grabbing the bo lock into shrinker, with a msm_gem_trylock() to
skip over bo's that are already locked.  This gets rid of the nested
lock classes.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>