drm/stm: ltdc: add layer alpha support

Android Hardware Composer supports alpha values applied to layers.
Enabling non-opaque layers for the STM CRTC could help offload GPU
resources for screen composition.

Signed-off-by: Raphael Gallais-Pou <[e-mail]>
Acked-by: Yannick Fertre <[e-mail]>
Reviewed-by: Yannick Fertre <[e-mail]>
Signed-off-by: Philippe Cornu <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/stm/ltdc.c

···
 			LXCFBLR_CFBLL | LXCFBLR_CFBP, val);
 
 	/* Specifies the constant alpha value */
-	val = CONSTA_MAX;
+	val = newstate->alpha >> 8;
 	reg_update_bits(ldev->regs, LTDC_L1CACR + lofs, LXCACR_CONSTA, val);
 
 	/* Specifies the blending factors */
···
 		return NULL;
 
 	drm_plane_helper_add(plane, &ltdc_plane_helper_funcs);
+
+	drm_plane_create_alpha_property(plane);
 
 	DRM_DEBUG_DRIVER("plane:%d created\n", plane->base.id);