drm/atomic: Constify mode argument to mode_valid_path()

The mode_valid_path() function validates the mode it receives without
ever modifying it. Constify the mode pointer argument to make that
explicit.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>