media: v4l2-fh: define v4l2_fh struct regardless of condition

v4l2_fh struct define differently by CONFIG_V4L2_MEM2MEM_DEV.
If some vendors use CONFIG_V4L2_MEM2MEM_DEV by module,
it can make the mismatch of v4l2_fh sturct.

By the mismatch, the following error occurs.
===============================
[    7.533506] v4l2_mem2mem: disagrees about version of symbol video_devdata
[    7.533594] v4l2_mem2mem: Unknown symbol video_devdata (err -22)
[    7.535319] v4l2_mem2mem: disagrees about version of symbol v4l2_event_pending
[    7.542532] v4l2_mem2mem: Unknown symbol v4l2_event_pending (err -22)
===============================

So v4l2_fh struct is modified to does not have dependency
for CONFIG_V4L2_MEM2MEM_DEV.

Signed-off-by: Seungchul Kim <sc377.kim@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>