drm: fix doc warnings in drm_atomic.h

Add description for parameters for
for_each_new_plane_in_state_reverse to fix warnings:

include/drm/drm_atomic.h:908: warning: Function parameter or member '__state' not described in 'for_each_new_plane_in_state_reverse'
include/drm/drm_atomic.h:908: warning: Function parameter or member 'plane' not described in 'for_each_new_plane_in_state_reverse'
include/drm/drm_atomic.h:908: warning: Function parameter or member 'new_plane_state' not described in 'for_each_new_plane_in_state_reverse'
include/drm/drm_atomic.h:908: warning: Function parameter or member '__i' not described in 'for_each_new_plane_in_state_reverse'

Fixes: a6c3c37b661d ("drm/amd/display: fix gcc set but not used warning of variable 'old_plane_state'")
Signed-off-by: Yu Kuai <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

include/drm/drm_atomic.h

···
 /**
  * for_each_new_plane_in_state_reverse - other than only tracking new state,
  * it's the same as for_each_oldnew_plane_in_state_reverse
+ * @__state: &struct drm_atomic_state pointer
+ * @plane: &struct drm_plane iteration cursor
+ * @new_plane_state: &struct drm_plane_state iteration cursor for the new state
+ * @__i: int iteration cursor, for macro-internal use
  */
 #define for_each_new_plane_in_state_reverse(__state, plane, new_plane_state, __i) \
 	for ((__i) = ((__state)->dev->mode_config.num_total_plane - 1);	\