drm/omap: rename dss_get_generic_clk_source_name()

Now that there is no "non-generic" version of the function to get the
clock source name, lets rename dss_get_generic_clk_source_name() to
dss_get_clk_source_name().

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