OMAP: DSS2: DISPC: remove finegrained clk enables/disables

dispc.c enables and disables clocks in almost every function to make
sure the clocks are enabled when the function is called. This is rather
unoptimal way to handle the problem.

With pm_runtime other components have to call dispc_runtime_get() to
enable dispc clocks before calling any other dispc functions. Thus the
finegrained clk enables/disables can be removed.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>