drm/i915: stop using is_cpu_edp() in intel_disable/post_disable_dp

Based on 3739850b - "drm/i915: disable the cpu edp port after the
cpu pipe" and the bspec disabling sequence for IVB and older it seems we
have to distinguish only the CPU vs. PCH port case, whether it's a DP or
eDP doesn't seem to matter. For IVB and older on the CPU side we can
only have eDP on port A, DP ports can only be on the PCH side. On VLV we
have only CPU side eDP/DP ports, no PCH. So the condition for the
disabling sequence we need for CPU ports is port == A || IS_VLV.

This allows us to remove is_cpu_edp() completely in a later patch.

v2:
- simplify (and fix) the condition for CPU side ports and adjust the
  commit message accordingly (Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>