drm/i915: simplify intel_crtc_driving_pch

By forking Ironlake and Haswell functions. The only callers are
{ironlake,haswell}_crtc_enable anyway, and this way we won't need to
add other checks on the Haswell version for the next gens.

V2: Even simpler, as pointed by Jani Nikula.

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