OMAPDSS: add omapdss_version

Add new enum, omapdss_version, that is used to tell which DSS hardware
version the SoC has. This enum is initialized during platform init, and
passed in the platform data to omapdss driver.

Note that the versions are not "continuous", that is, you cannot check
if the version is less or greater than something, but you need to check
for exact version match. In other words, this is invalid:

/* test if DSS is 3630 or earlier */
if (ver <= OMAPDSS_VER_OMAP3630)
	...

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