drm/crtc: constify drm_crtc_mask parameter

Now that drm_crtc_index takes a const, the same can be done for drm_crtc_mask.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/6e32d013-6fee-34ad-f8d2-59139f3dc4c1@linux.intel.com