drm/omap: remove dss_feat_get_clk_source_name()

We have two functions to return a name for clock sources for debugging
purposes: dss_feat_get_clk_source_name() and
dss_get_generic_clk_source_name().

The former is supposed to return a DSS IP version specific name for the
clock source, and the latter is supposed to return a more generic name.

All this seems a bit pointless, so let's remove the former one.

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