OMAP: DSS2: DISPC: rename manager related funcs

Rename dispc's manager 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 This meaning is no longer valid.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_mgr_*
- Remove "channel" from the name, e.g. dispc_enable_channel ->
  dispc_mgr_enable

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

All dispc_mgr_* functions have enum omap_channel as the first parameter.

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