drm/amd/display: Fix potential memory leak

[Why]
vblank_workqueue is never released.

[How]
Free it upon dm finish.
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -1226,6 +1226,15 @@ static void amdgpu_dm_fini(struct amdgpu_device *adev)
 	if (adev->dm.dc)
 		dc_deinit_callbacks(adev->dm.dc);
 #endif
+
+#if defined(CONFIG_DRM_AMD_DC_DCN)
+	if (adev->dm.vblank_workqueue) {
+		adev->dm.vblank_workqueue->dm = NULL;
+		kfree(adev->dm.vblank_workqueue);
+		adev->dm.vblank_workqueue = NULL;
+	}
+#endif
+
 	if (adev->dm.dc->ctx->dmub_srv) {
 		dc_dmub_srv_destroy(&adev->dm.dc->ctx->dmub_srv);
 		adev->dm.dc->ctx->dmub_srv = NULL;