drm/i915: fork a Haswell version of ironlake_crtc_{enable, disable}

The way we enable and disable the PCH on Haswell changed considerably
since now we have only one PCH transcoder, so we can't keep the same
asserts and we also can't just unconditionally disable the PCH
transcoder for non-PCH outputs. So let's fork a Haswell version.

These new functions look exactly the same as the ironlake versions.
The next patches will introduce the differences.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>