drm: Turn bus flags macros into an enum

This allows nicer kerneldoc with an easy way to reference the enum and
the values.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>