OMAPDSS: DISPC: rename dispc_mgr_enable/disable to _sync

The current dispc_mgr_enable/disable function are blocking, and do a bit
too much for omapdrm. We'll expose new enable & disable functions that
will just set the bits in the registers in the following patches.

This patch renames the current functions to *_sync, to make it clear
that they are blocking, and also to free up the dispc_mgr_enable/disable
names for these new functions.

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