OMAPDSS: hide dss_select_dispc_clk_source()

dss.c currently exposes functions to configure the dispc source clock
and lcd source clock. There are configured separately from the output
drivers.

However, there is no safe way for the output drivers to handle dispc
clock, as it's shared between the outputs. Thus, if, say, the DSI driver
sets up DSI PLL and configures both the dispc and lcd clock sources to
that DSI PLL, the resulting dispc clock could be too low for, say, HDMI.

Thus the output drivers should really only be concerned about the lcd
clock, which is what the output drivers actually use. There's lot to do
to clean up the dss clock handling, but this patch takes one step
forward and removes the use of dss_select_dispc_clk_source() from the
output drivers.

After this patch, the output drivers only configure the lcd source
clock. On omap4+ the dispc src clock is never changed from the default
PRCM source. On omap3, where the dispc and lcd clocks are actually the
same, setting the lcd clock source sets the dispc clock source.

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