OMAPDSS: move 'compatible' converter to omapdss driver

Move the panel/encoder driver compatible-string converter from
arch/arm/mach-omap2/display.c to omapdss driver. That is a more logical
place for it, as it's really an omapdss internal hack.

The code is rewritten to follow the video node graph, starting from
omapdss. This removes the need to have the device compatible-string
database.

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