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>