drm/i915/pxp: prefer forward declaration over includes

Always use forward declarations instead of includes in headers if
possible.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214173644.2097124-1-jani.nikula@intel.com