drm/i915: transform HAS_PCH_SPLIT in a feature check

The macro is becoming too complex and with VLV upon us it can lead to
confusion. So transforming this into a feature check instead.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
[danvet: fixed conflict with is_valleyview addition.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>