media: rcar-vin: Rename VNDMR_DTMD_ARGB1555 to VNDMR_DTMD_ARGB

The value have nothing to do with ARGB1555, it controls if the alpha
component should be filled in for ARGB1555 or ARGB888. Rename it to
reflect this.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>