videomode: create enum for videomode's display flags

Instead of having plain defines for the videomode's flags, add an enum
for the flags. This makes the flags clearer to use, as the enum tells
which values can be used with the flags field.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>