OMAP: DSS2: RFBI: clock enable/disable changes

RFBI enables and disables clocks inside almost every function to get a
finegrained control to the clocks. However, the current understanding is
that this is not necessary power-management-wise.

Change the clocking scheme so that RFBI clocks are enabled when the
omapdss_rfbi_display_enable is called, and disabled when
omapdss_rfbi_display_disable is called.

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