drm/imx: switch to use media bus formats

imx-drm internally misused the V4L2_PIX_FMT constants, which are supposed to
describe the pixel format of frame buffers in memory, to describe the pixel
format on the bus between the display controller and the encoder hardware.
Now that MEDIA_BUS_FMT constants are available to drm drivers, use those
instead.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Emil Renner Berthing <kernel@esmil.dk>