videomode: combine videomode dmt_flags and data_flags

Both videomode and display_timing contain flags describing the modes.
These are stored in dmt_flags and data_flags. There's no need to
separate these flags, and having separate fields just makes the flags
more difficult to use.

This patch combines the fields and renames VESA_DMT_* flags to
DISPLAY_FLAGS_*.

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