OMAPDSS: DISPC: Change return type of dispc_mgr_set_clock_div()

dipsc_mgr_set_clock div has an int return type to report errors or success.
The function doesn't really check for errors and always returns 0. Change
the return type to void.

Checking for the correct DISPC clock divider ranges will be done when a DSS2
user does a manager apply. This support will be added later.

Signed-off-by: Archit Taneja <archit@ti.com>