drm/vbox: Use offset-adjusted shadow-plane mappings

For framebuffers with non-zero offset fields, shadow-plane helpers
provide a pointer to the first byte of the contained data. Use it in
vbox.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Acked-by: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vboxvideo/vbox_mode.c

···
 	u32 height = new_state->crtc_h;
 	struct drm_shadow_plane_state *shadow_plane_state =
 		to_drm_shadow_plane_state(new_state);
-	struct dma_buf_map map = shadow_plane_state->map[0];
+	struct dma_buf_map map = shadow_plane_state->data[0];
 	u8 *src = map.vaddr; /* TODO: Use mapping abstraction properly */
 	size_t data_size, mask_size;
 	u32 flags;