drm/vmwgfx/vmwgfx_blit: Add description for 'vmw_bo_cpu_blit's 'diff' param

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

 drivers/gpu/drm/vmwgfx/vmwgfx_blit.c:452: warning: Function parameter or member 'diff' not described in 'vmw_bo_cpu_blit'

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: Zack Rusin <zackr@vmware.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181601.3432599-8-lee.jones@linaro.org

drivers/gpu/drm/vmwgfx/vmwgfx_blit.c

@@ -431,6 +431,7 @@ static int vmw_bo_cpu_blit_line(struct vmw_bo_blit_line_data *d,
  * @src_stride: Source stride in bytes.
  * @w: Width of blit.
  * @h: Height of blit.
+ * @diff: The struct vmw_diff_cpy used to track the modified bounding box.
  * return: Zero on success. Negative error value on failure. Will print out
  * kernel warnings on caller bugs.
  *