drm/i915: stop using is_pch_edp() in is_cpu_edp()

is_pch_edp() will be removed by the next patch, so replace it by a check
for the port and device type.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>