OMAP: DSS2: DSI: implement enable/disable SCP clk

SCP clock is needed for CIO on OMAP3, and for CIO and PLL on OMAP4.
Current driver enables the CIO clock always when DSI display is
initialized. However, if a DPI display tries to use DSI PLL, the SCP
clock is never enabled.

This patch implements simple ref counting enable/disable functions for
SCP clock.

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