drm/radeon/radeon_sync: Add description for function param 'rdev'

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

 drivers/gpu/drm/radeon/radeon_sync.c:92: warning: Function parameter or member 'rdev' not described in 'radeon_sync_resv'

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: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_sync.c

@@ -79,6 +79,7 @@ void radeon_sync_fence(struct radeon_sync *sync,
 /**
  * radeon_sync_resv - use the semaphores to sync to a reservation object
  *
+ * @rdev: radeon_device pointer
  * @sync: sync object to add fences from reservation object to
  * @resv: reservation object with embedded fence
  * @shared: true if we should only sync to the exclusive fence