[media] Move mediabus format definition to a more standard place

Define MEDIA_BUS_FMT macros (re-using the values defined in the
v4l2_mbus_pixelcode enum) into a separate header file so that they can be
used from the DRM/KMS subsystem without any reference to the V4L2
subsystem.

Then set V4L2_MBUS_FMT definitions to the MEDIA_BUS_FMT values using the
V4L2_MBUS_FROM_MEDIA_BUS_FMT macro.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>