drm/i915/gt: Drop duplicate register definition for VDBOX_CGCTL3F18

Due to some mistaken merge conflict resolution, we wound up with a copy
of VDBOX_CGCTL3F18 in both intel_engine_regs.h and intel_gt_regs.h.
Since this is a per-engine register, referenced relative to an engine's
base offset, drop the copy from intel_gt_regs.h

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209051140.1599643-2-matthew.d.roper@intel.com