drm/i915: Extract intel_crtc_pch_transcoder()

Extract the code to determine which PCH transcoder we're using to a
small helper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476464574-32230-2-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>