drm/atomic: Constify drm_atomic_crtc_needs_modeset()

drm_atomic_crtc_needs_modeset() doesn't change the passed in
crtc state, so pass it as const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480009622-28127-1-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>