drm/xe: Protect devcoredump access after unbind

While we don't have the full flow protection when devcoredump
is accessed after device unbind. Let's at least for now
protect against null dereference:

[  422.766508] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[  423.119584] RIP: 0010:xe_vm_snapshot_free+0x30/0x180 [xe]

While at it, I also fixed a non-standard code-declaration block
on the similar function of xe_guc_submit.

v2: - Use IS_ERR_OR_NULL (Nirmoy)
    - Expand to other functions

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240403195044.239766-1-rodrigo.vivi@intel.comSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_guc_submit.c

@@ -1928,6 +1928,7 @@ xe_guc_exec_queue_snapshot_print(struct xe_guc_submit_exec_queue_snapshot *snaps
 void xe_guc_exec_queue_snapshot_free(struct xe_guc_submit_exec_queue_snapshot *snapshot)
 {
 	int i;
+
 	if (!snapshot)
 		return;
 

drivers/gpu/drm/xe/xe_vm.c

@@ -3426,7 +3426,7 @@ struct xe_vm_snapshot *xe_vm_snapshot_capture(struct xe_vm *vm)
 
 void xe_vm_snapshot_capture_delayed(struct xe_vm_snapshot *snap)
 {
-	if (IS_ERR(snap))
+	if (IS_ERR_OR_NULL(snap))
 		return;
 
 	for (int i = 0; i < snap->num_snaps; i++) {
@@ -3483,7 +3483,7 @@ void xe_vm_snapshot_print(struct xe_vm_snapshot *snap, struct drm_printer *p)
 {
 	unsigned long i, j;
 
-	if (IS_ERR(snap)) {
+	if (IS_ERR_OR_NULL(snap)) {
 		drm_printf(p, "[0].error: %li\n", PTR_ERR(snap));
 		return;
 	}
@@ -3514,7 +3514,7 @@ void xe_vm_snapshot_free(struct xe_vm_snapshot *snap)
 {
 	unsigned long i;
 
-	if (IS_ERR(snap))
+	if (IS_ERR_OR_NULL(snap))
 		return;
 
 	for (i = 0; i < snap->num_snaps; i++) {