drm/i915/pc8: Add parent interface for PC8 forcewake tricks

drm/i915/pc8: Add parent interface for PC8 forcewake tricks

We use forcewake to prevent the SoC from actually entering
PC8 while performing the PC8 disable sequence. Hide that
behind a new parent interface to eliminate the naked
forcewake/uncore usage from the display power code.

v2: Mark the interface optional and warn if
    someone calls it when not provided (Jani)
    Include the header to make sure the extern
    declaration matches the definition (Jani)
v3: Rebase due to shuffling

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251218182052.18756-1-ville.syrjala@linux.intel.com