OMAP: DSS2: DISPC: rename overlay related funcs

Rename dispc's overlay related functions as follows:

- Remove prepending underscores, which were originally used to inform
  that the clocks needs to be enabled. This meaning is no longer valid.
- Prepend the functions with dispc_ovl_*
- Remove "plane" from the name, e.g. dispc_set_plane_ba0 ->
  dispc_ovl_set_ba0

The idea is to group overlay related functions so that it can be deduced
from the function name that it writes to overlay spesific registers.

All dispc_ovl_* functions have enum omap_plane as the first parameter.

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