drm/i915/dp: remove redundant is_pch_edp checks

If is_edp is true, is_pch_edp will always be true.  So limit the calls
to the latter function to places where the distinction actually matters.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>