videomode: don't allocate mem in of_get_display_timing()

Move the allocation of display_timing memory from of_get_display_timing() to
of_get_display_timings(). This allows us to use of_get_display_timing()
in a way that doesn't require dynamic memory allocation.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>