drm/ssd130x: Synchronize access to dma-buf imported GEM BOs

Synchronize CPU access to GEM BOs with other drivers when updating the
screen buffer. Imported DMA buffers might otherwise contain stale data.

Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220927095249.1919385-1-javierm@redhat.com