drm/omap: cleanup OMAP_BO_SCANOUT use

omap_gem_new() has a comment about OMAP_BO_SCANOUT which does not make
sense. Also, for the TILER case, we drop OMAP_BO_SCANOUT flag for some
reason.

It's not clear what the original purpose of OMAP_BO_SCANOUT is, but
presuming it means "scanout buffer, something that can be consumed by
DSS", this patch cleans up the above issues.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010120000.1421-7-jjhiblot@ti.com