drm/meson: Add support for XBGR8888 & ABGR8888 formats

Add missing XBGR8888 & ABGR8888 formats variants from the primary plane.

Fixes: bbbe775e ("drm: Add support for Amlogic Meson Graphic Controller")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190429075238.7884-1-narmstrong@baylibre.com