drm: Add drm_modeset_lock_assert_held()

Add a small wrapper around lockdep_assert_held() to make
it a bit more conventinet to use with modeset locks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708125325.16576-2-ville.syrjala@linux.intel.com


Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>