drm/i915/dp: Refactor common eDP lid detection

Both gmch and pch detection routines used the exact same routine for
eDP, so de-duplicate.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: : Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>