drm/msm/gem: Add obj->lock wrappers

This will make it easier to transition over to obj->resv locking for
everything that is per-bo locking.

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