drm/xe: Don't support execlists in xe_gt_tlb_invalidation layer

The xe_gt_tlb_invalidation layer implements TLB invalidations for a GuC
backend. Simply return if in execlists mode. A follow up may properly
implement the xe_gt_tlb_invalidation layer for both GuC and execlists.

Fixes: a9351846 ("drm/xe: Break of TLB invalidation into its own file")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222232021.3911545-4-matthew.brost@intel.com