OMAP: DSS2: OMAPFB: Add support for switching memory regions

Separate the memory region from the framebuffer device a little bit.
It's now possible to select the memory region used by the framebuffer
device using the new mem_idx parameter of omapfb_plane_info. If the
mem_idx is specified it will be interpreted as an index into the
memory regions array, if it's not specified the framebuffer's index is
used instead. So by default each framebuffer keeps using it's own
memory region which preserves backwards compatibility.

This allows cloning the same memory region to several overlays and yet
each overlay can be controlled independently since they can be
associated with separate framebuffer devices.

Signed-off-by: Ville Syrjälä <ville.syrjala@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>