drm/radeon/radeon_mn: Supply description for 'cur_seq' even if it is unused

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/radeon_mn.c:51: warning: Function parameter or member 'cur_seq' not described in 'radeon_mn_invalidate'

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_mn.c

@@ -41,6 +41,7 @@
  *
  * @mn: our notifier
  * @range: the VMA under invalidation
+ * @cur_seq: Value to pass to mmu_interval_set_seq()
  *
  * We block for all BOs between start and end to be idle and
  * unmap them by move them into system domain again.