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: a6c3c37b ("drm/amd/display: fix gcc set but not used warning of variable 'old_plane_state'")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>