media: Replace file->private_data access with custom functions

media: Replace file->private_data access with custom functions

Accessing file->private_data manually to retrieve the v4l2_fh pointer is
error-prone, as the field is a void * and will happily cast implicitly
to any pointer type.

Replace all remaining locations that read the v4l2_fh pointer directly
from file->private_data and cast it to driver-specific file handle
structures with driver-specific functions that use file_to_v4l2_fh() and
perform the same cast.

No functional change is intended, this only paves the way to remove
direct accesses to file->private_data and make V4L2 drivers safer.
Other accesses to the field will be addressed separately.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>