drm/msm/gem: Switch over to obj->resv for locking

This also converts the special msm_gem_get_vaddr_active() to expect the
lock to already be held.  There are two call-sites for this, one already
has the lock held, so it is more straightforward to just open-code the
locking for the other caller.

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