drm/i915: don't use HPD_PORT_A as an alias for HPD_NONE

Currently HPD_PORT_A is used as an alias for HPD_NONE to mean that the
given port doesn't support long/short HPD pulse detection. SDVO and CRT
ports are like this and for these ports we only want to know whether an
hot plug event was detected on the corresponding pin. Since at least on
BXT we need long/short pulse detection on PORT A as well (added by the
next patch) remove this aliasing of HPD_PORT_A/HPD_NONE and let the
return value of intel_hpd_pin_to_port() show whether long/short pulse
detection is supported on the passed in pin.

No functional change.

v2:
- rebase on top of -nightly (Daniel)
- make the check for intel_hpd_pin_to_port() return value more readable
  (Sivakumar)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>