drm/i915/gen9_bc: Introduce HPD pin mappings for TGP PCH + CML combos

Next, let's start introducing the HPD pin mappings for Intel's new gen9_bc
platform in order to make hotplugging display connectors work. Since
gen9_bc is just a TGP PCH along with a CML CPU, except with the same HPD
mappings as ICL, we simply add a skl_hpd_pin function that is shared
between gen9 and gen9_bc which handles both the traditional gen9 HPD pin
mappings and the Icelake HPD pin mappings that gen9_bc uses.

Changes since v4:
* Split this into its own commit
* Introduce skl_hpd_pin() like vsyrjala suggested and use that instead of
  sticking our HPD pin mappings in TGP code

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
[originally from Tejas's work]
Signed-off-by: Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209212832.1401815-4-lyude@redhat.com