drm/i915/pch: Remove unused i915->pch_id

With the PCH checks based on PCH types instead of IDs, the i915->pch_id
member has become unused. Remove it.

Reviewed-by: Nemesa Garg <nemesa.garg@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fac1c59800128e8f398e83d718a3a5dc235d0526.1738923308.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>