drm/i915: drop gt/intel_gt.h include from skl_universal_plane.c

No longer needed after commit 94bcf876

 ("drm/i915/mtl: Drop
Wa_14017240301").

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/20231011142704.985867-1-jani.nikula@intel.com