omapdss: HDMI: move common functions to a separate file

The OMAP4 HDMI encoder driver(hdmi4.c) contains timings tables, and helper
functions which can be used as is by the OMAP5/DRA7x encoder driver. Move these
to hdmi_common.c so that it's not replicated in the future.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>