drm/i915: Clarfify the DP code platform checks

intel_dp.c is a mess with all the checks for different
platform/PCH variants and ports. Try to clean it up by recognizing
the following facts:
- IVB port A, and CPT port B/C/D are always the special cases
- VLV/CHV don't have port A
- Using the same kind of logic everywhere makes things much easier to
  parse

So let's move the IVB port A and PCH port B/C/D checks to be done first,
and let the other cases fall through, and always check for these things
using the same logic.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>