media: vimc: free vimc_cap_device when the last user disappears

Don't free vimc_cap_device immediately, instead do this
in the video_device release function which is called when the
last user closes the video device. Only then is it safe to
free the memory.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>