drm: rcar-du: Fix r8a779a0 color issue

The rcar DU driver on r8a779a0 has a bug causing some specific colors
getting converted to transparent colors, which then (usually) show as
black pixels on the screen.

The reason seems to be that the driver sets PnMR_SPIM_ALP bit in
PnMR.SPIM field, which is an illegal setting on r8a779a0. The
PnMR_SPIM_EOR bit also illegal.

Add a new feature flag for this (lack of a) feature and make sure the
bits are zero on r8a779a0.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>