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