OMAPDSS: setup default dss fck

We don't currently set the dss fck when starting up. This is not a
problem, as we setup the fck later when configuring the pixel clocks. Or
this is how it was for omap2, for the rest of the omaps this may not be
so.

For DSI, HDMI and also for DPI when using DSI PLL, we don't need to
change the dss fck, and thus it may be left unconfigured. Usually the
dss fck is already setup fine by default, but we can't trust this.

This patch sets the dss fck to maximum at probe time.

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