drm/i915: drop TVclock special casing on ilk+

TV-out uses the same reference clock as everyone else. The only
difference seems to be in the slightly different CB tuning limit.

Note that PLL_REF_INPUT_TVCLKINBC is a reserved value on ilk+. Also
strictly speaking we don't support native TV-out on ilk+, hence all
that code is dead. But Bspec still contains some residual mentions of
native TV-out on some pch-split platforms, so I've figured it doesn't
hurt to keep the code around a bit longer (e.g. in the cb tune
function).

v2: Improve the commit message as Jani suggested in his review.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>