drm/udl: Begin/end access to imported buffers in damage-handler

The damage-handler code now invokes dma_buf_{begin,end}_access()
for imported buffers. These calls were missing from the page-flip
and modesetting code paths. The patch also fixes an bug in the
original where an error code was overwritten by the result of
dma_buf_end_cpu_access().

v2:
	* only return an error code from dma_buf_end_cpu_access() if
	  no other error code has been set before

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191206085954.9697-7-tzimmermann@suse.de