drm/i915: clarify IBX dp workaround

Instead of checking for !CPT, check for IBX to make it clearer that
this is a IBX-specific workaround. No functional change because we
smash the PPT PCH into the HAS_PCH_CPT check and atm DP isn't enabled
on the haswell LPT PCH yet.

See Bspec Vol 3, Part 3, Section 4.[3-5].1 about DP[BCD], bit 30 for
details of this workaround.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>