drm: rcar-du: Fix Alpha blending issue on Gen3

As per R-Car-Gen3_Common_OPC_Customer_Notifications_V30.1.pdf,
unexpected image output(such as incorrect colors or planes being
invisible) can happen on the below conditions, as PnALPHAR register
is not initialized by reset.

When alpha blending (PpMRm.PpSPIM=0b101) is performed and:
	•two Planes are enabled on any DUn (n=0,1,2,3)
		oDSPRn= 0x0000 0031 or 0x0000 0013
	•or DU0 and DU1 is used for display at the same time
		oDSPR0= 0x0000 0001 and DSPR1= 0x0000 0003
		oDSPR0= 0x0000 0003 and DSPR1= 0x0000 0001
	•or DU2 and DU3(H3 Only) is used for display at the same time
		oDSPR2= 0x0000 0001 and DSPR3= 0x0000 0003
		oDSPR2= 0x0000 0003 and DSPR3= 0x0000 0001

This patch set PnALPHAR register to 0 to avoid this issue.

Signed-off-by: LUU HOAI <hoai.luu.ub@renesas.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>