drm: vc4: enable XBGR8888 and ABGR8888 pixel formats

DRM_FORMAT_XBGR8888 and DRM_FORMAT_ABGR8888 are 2 of the native formats
used in Android, so enable them for VC4. There seems to be no logic behind
HVS_PIXEL_ORDER_xxxx naming, but HVS_PIXEL_ORDER_ARGB seems to work
correctly.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Eric Anholt <eric@anholt.net>