drm: Add explicit acquire ctx handling around ->gamma_set

Just the groundwork to prepare for adding the acquire cxt parameter to
the ->gamma_set hook. Again we need a temporary hack to fill out
mode_config.acquire_ctx until the atomic helpers are switched over.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-12-daniel.vetter@ffwll.ch