drm/atomic: Consitfy mode parameter to drm_atomic_set_mode_for_crtc()

drm_atomic_set_mode_for_crtc() doesn't modify the passed mode, so let's
make it const.

Signed-off-by: Ville Syrjälä <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/drm_atomic.c

···
  * Zero on success, error code on failure. Cannot return -EDEADLK.
  */
 int drm_atomic_set_mode_for_crtc(struct drm_crtc_state *state,
-				 struct drm_display_mode *mode)
+				 const struct drm_display_mode *mode)
 {
 	struct drm_mode_modeinfo umode;
 

include/drm/drm_atomic.h

···
 
 int __must_check
 drm_atomic_set_mode_for_crtc(struct drm_crtc_state *state,
-			     struct drm_display_mode *mode);
+			     const struct drm_display_mode *mode);
 int __must_check
 drm_atomic_set_mode_prop_for_crtc(struct drm_crtc_state *state,
 				  struct drm_property_blob *blob);