OMAPDSS: remove partial update from the overlay manager

Partial update for manual update displays has never worked quite well:
* The HW has limitations on the update area, and the x and width need to
  be even.
* Showing a part of a scaled overlay causes artifacts.
* Makes the management of dispc very complex

Considering the above points and the fact that partial update is not
used anywhere, this and the following patches remove the partial update
support. This will greatly simplify the following re-write of the apply
mechanism to get proper locking and additional features like fifo-merge.

This patch removes the partial update from the manager.c.

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