drm/i915/display: no need for gt/gen8_ppgtt.h

Remove an unnecessary include.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208133638.478024-1-jani.nikula@intel.com