drm/i915: unify HDMI/DP hpd definitions

They're physically the same pins and also the same bits, duplicating
only confuses the reader. This also makes it a bit obvious that we
have quite some code duplication going on here. Squashing that is for
a larger rework in our hpd handling though.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>