drm/i915/gt: Potential error pointer dereference in pinned_context()

If the intel_engine_create_pinned_context() function returns an error
pointer, then dereferencing "ce" will Oops.  Use "vm" instead of
"ce->vm".

Fixes: cf586021 ("drm/i915/gt: Pipelined page migration")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210813113600.GC30697@kili

drivers/gpu/drm/i915/gt/intel_migrate.c

@@ -177,7 +177,7 @@ static struct intel_context *pinned_context(struct intel_gt *gt)
 	ce = intel_engine_create_pinned_context(engine, vm, SZ_512K,
 						I915_GEM_HWS_MIGRATE,
 						&key, "migrate");
-	i915_vm_put(ce->vm);
+	i915_vm_put(vm);
 	return ce;
 }