staging: drm/imx: fix RGB formats, make ipu_cpmem_set_fmt take a drm_fourcc

The drm fourccs define formats not available as video4linux pixel formats,
such as DRM_FORMAT_BGR565, or the DRM_FORMAT_RGBX/BGRX variants.
Also, contrary to the v4l2 formats, the drm formats are well defined.

This patch also fixes the BGRA32 and RGB/RGB24 internal formats to use a
common internal representation.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>