drm/i915: Do not cleanup obj with NULL bo->resource

For delayed BO release i915_ttm_delete_mem_notify()
gets called twice, once with proper bo->resource and
another time with NULL. We shouldn't do anything for
the 2nd time as we already cleaned up the obj once.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/6850
Fixes: ad74457a ("drm/i915/dgfx: Release mmap on rpm suspend")
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220920170628.3391-1-nirmoy.das@intel.com

drivers/gpu/drm/i915/gem/i915_gem_ttm.c

@@ -550,7 +550,7 @@ static void i915_ttm_delete_mem_notify(struct ttm_buffer_object *bo)
 	struct drm_i915_gem_object *obj = i915_ttm_to_gem(bo);
 	intel_wakeref_t wakeref = 0;
 
-	if (likely(obj)) {
+	if (bo->resource && likely(obj)) {
 		/* ttm_bo_release() already has dma_resv_lock */
 		if (i915_ttm_cpu_maps_iomem(bo->resource))
 			wakeref = intel_runtime_pm_get(&to_i915(obj->base.dev)->runtime_pm);