We don't have any locking in camerarx for the subdev ops. We have
managed fine so far without locking, but in the future multiple video
capture devices can use the same camerarx, and locking is a must.

Add a mutex to protect the camerarx subdev ops. Some of the functions
were slightly restructured to make lock handling cleaner.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>