drm/msm/gem: Add some _locked() helpers

When we cut-over to using dma_resv_lock/etc instead of msm_obj->lock,
we'll need these for the submit path (where resv->lock is already held).

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