drm/i915: move enum hpd_pin to intel_display.h

It's not the ideal location, but a better alternative than
i915_drv.h. The goal is to break the intel_display_types.h to i915_drv.h
dependency.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9f882eff78cdc6b28c18e73f5e53f57e413240dc.1638366969.git.jani.nikula@intel.com