drm/mediatek: Turn off Alpha bit when plane format has no alpha

This change enables XR24 format to be displayed as an overlay on top of
the primary plane.

Suggested-by: Sean Paul <seanpaul@chromium.org>
To: CK Hu <ck.hu@mediatek.com>
To: dri-devel@lists.freedesktop.org
Cc: Daniele Castagna <dcastagna@chromium.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Mark Yacoub <markyacoub@google.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>