OMAP2, 3: DSS2: Create new file display.c for central dss driver registration.

A new file display.c is introduced for display driver init, which adds a function
omap_display_init to do the DSS driver registration. This is the first step in moving
away registration of DSS from board files into a common place.

Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Senthilvadivu Guruswamy <svadivu@ti.com>
Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>