OMAP: DSS2: remove unneeded fck enable/disables

Now that the HWMOD fmwk handles the fcks of DSS modules properly, the
DSS driver no longer needs to explicitely enable/disable the fck.

This patch removes the enables/disables of fck from dispc, dsi and dss.
The clk_get(fck) is still needed there, as the modules need to know the
frequency of the clock.

For hdmi and venc this patch also removes the clk_get(fck), as they
don't need the clock at all.

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