drm/omap: cleanup DPI clock source handling

We can clean up the DPI driver's clock source handling by using the
dss_clk_source instead of only a dss_pll pointer.

This will also make it possible to use additional clock sources, like
PLL1_3 or HDMI_PLL, which the code did not support earlier.

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