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 <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170518193837.393-1-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>