drm/i915: Fix oops due to missing stack depot

We call __save_depot_stack() unconditionally so the stack depot
must always be initialized or else we'll oops on platforms without
runtime pm support.

Presumably we've not seen this in CI due to stack_depot_init()
already getting called via drm_mm_init()+CONFIG_DRM_DEBUG_MM.

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com> # stackdepot
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Fixes: 2dba5eb1 ("lib/stackdepot: allow optional init and stack_table allocation by kvmalloc()")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220126081539.23227-1-ville.syrjala@linux.intel.comAcked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Imre Deak <imre.deak@intel.com>

drivers/gpu/drm/i915/intel_runtime_pm.c

@@ -68,9 +68,7 @@ static noinline depot_stack_handle_t __save_depot_stack(void)
 static void init_intel_runtime_pm_wakeref(struct intel_runtime_pm *rpm)
 {
 	spin_lock_init(&rpm->debug.lock);
-
-	if (rpm->available)
-		stack_depot_init();
+	stack_depot_init();
 }
 
 static noinline depot_stack_handle_t