drm/omap: add PLL helper funcs

Add two PLL helper functions:

dss_pll_find_by_src() which returns the dss_pll for the given
dss_clk_source.

dss_pll_get_clkout_idx_for_src() which returns the clkout index for the
given dss_clk_source.

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