videomode: videomode_from_timing work

We currently have videomode_from_timing(), which takes one
display_timing entry from display_timings.

To make it easier to use display_timing without display_timings, this
patch renames videomode_from_timing() to videomode_from_timings(), and
adds a new videomode_from_timing() which just converts a given
display_timing to videomode.

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