drm/mediatek: Add RGB[A] variants to published plane formats

These formats are handled in the rdma code, but for some reason they're
not published as supported formats for the planes. So add them to the
list.

Cc: Nicolas Boichat <drinkcat@chromium.org>
Cc: Daniele Castagna <dcastagna@chromium.org>
Cc: Miguel Casas <mcasas@chromium.org>
Tested-by: Miguel Casas <mcasas@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>