drm/i915: duplicate intel_enable_pll into i9xx and vlv versions

Mostly since I _really_ don't want to touch the vlv hell.

No code change, just duplication. Also kill a now seriously outdated
code comment - the remark about the dvo encoder is now handled with
the pipe A quirk.

v2: Update the BUG_ONs as suggested by Jani (both in vlv_ and i9xx_
functions, since the split happens here).

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>