drm/i915: garbage-collect vlv refclk function

Simply inline the 100MHz default we're using. Having gunk around that
has leftover LVDS support on a platform that just doesn't have this
isn't of any use.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>