[media] media: Move media_get_uptr() macro out of the media.h user space header

The media_get_uptr() macro is mostly useful only for the IOCTL handling
code in media-device.c so move it there.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>