drm/i915: Fix possible null pointer dereference after drm_dbg_printer conversion

Request can be NULL if no guilty request was identified so simply use
engine->i915 instead.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: d50892a ("drm/i915: switch from drm_debug_printer() to device specific drm_dbg_printer()")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Luca Coelho <luciano.coelho@intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20240219131423.1854991-1-tvrtko.ursulin@linux.intel.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>

Author: tursulin

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

@@ -96,8 +96,8 @@ static void heartbeat_commit(struct i915_request *rq,
 static void show_heartbeat(const struct i915_request *rq,
 			   struct intel_engine_cs *engine)
 {
-	struct drm_printer p = drm_dbg_printer(&rq->i915->drm, DRM_UT_DRIVER,
-					       "heartbeat");
+	struct drm_printer p =
+		drm_dbg_printer(&engine->i915->drm, DRM_UT_DRIVER, "heartbeat");
 
 	if (!rq) {
 		intel_engine_dump(engine, &p,