drm/i915: make DP configuration vars less confusing in ironlake_crtc_mode_se

Both PCH and CPU eDP are DP, so set the is_dp flag to true.  Add
is_cpu_edp and is_pch_edp bools to make checking for each less verbose
(rather than has_edp_encoder && !intel_encoder_is_pch_edp() sprinkled
everywhere).  And rename the "has_edp_encoder" variable to just
"edp_encoder".

With the above variables cleaned up, the rest of the code becomes a bit
more readable and clear.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>