media: bdisp: Add missing check for create_workqueue

Add the check for the return value of the create_workqueue
in order to avoid NULL pointer dereference.

Fixes: 28ffeebb ("[media] bdisp: 2D blitter driver using v4l2 mem2mem framework")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>