drm/omap: fix DPI clkout index handling

DPI driver uses a fixed clkout index, 0. This has worked fine as the
first clkout is usually used.

To generalize the code and to support additional clock sources, change
the code to use dss_pll_get_clkout_idx_for_src() to get the clkout
index.

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