drm/omap: copy enum omap_dss_clk_source

At the moment 'enum omap_dss_clk_source' is in omapdss.h, shared by
omapdrm and omapfb. We're about to improve the omapdrm clock code, so we
need to make a separate copy of the enum for each driver.

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