drm/omap: fix fbdev pix format to support all platforms

omap_fbdev always creates a framebuffer with ARGB8888 pixel format. On
OMAP3 we have VIDEO1 overlay that does not support ARGB8888, and on
OMAP2 none of the overlays support ARGB888.

This patch changes the omap_fbdev's fb to XRGB8888, which is supported
by all platforms.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

drivers/gpu/drm/omapdrm/omap_fbdev.c

@@ -112,11 +112,8 @@ static int omap_fbdev_create(struct drm_fb_helper *helper,
 	dma_addr_t paddr;
 	int ret;
 
-	/* only doing ARGB32 since this is what is needed to alpha-blend
-	 * with video overlays:
-	 */
 	sizes->surface_bpp = 32;
-	sizes->surface_depth = 32;
+	sizes->surface_depth = 24;
 
 	DBG("create fbdev: %dx%d@%d (%dx%d)", sizes->surface_width,
 			sizes->surface_height, sizes->surface_bpp,