OMAPDSS: remove vaddr from overlay info

overlay_info struct, used to configure overlays, currently includes both
physical and virtual addresses for the pixels. The vaddr was added to
support more exotic configurations where CPU would be used to update a
display, but it is not currently used and there has been no interest in
the feature. Using CPU to update a screen is also less interesting now
that OMAP4 has two LCD outputs.

This patch removes the vaddr field, and modifies the users of omapdss
accordingly. This makes the use of omapdss a bit simpler, as the user
doesn't need to think if it needs to give the vaddr.

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