drm/omap: cleanup OMAP_BO flags

Reorder OMAP_BO flags and improve the comments.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010120000.1421-5-jjhiblot@ti.com