drm/msm/gem: Add msm_gem_assert_locked()

All use of msm_gem_is_locked() is just for WARN_ON()s, so extract out
into an msm_gem_assert_locked() patch.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/496136/
Link: https://lore.kernel.org/r/20220802155152.1727594-15-robdclark@gmail.com