drm/omapdrm: Set fbdev FBINFO_VIRTFB flag

Mark the framebuffer with FBINFO_VIRTFB. The framebuffer range is
in DMA-able memory and should be accessed with the CPU's regular
memory ops.

v2:
	* drop FBINFO_DEFAULT

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Cc: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230707083422.18691-11-tzimmermann@suse.de