drm/amd/display: Remove redundant safeguards for dmub-srv destroy()

[Why]
dc_dmub_srv_destroy() has internal null-check and null assignment.
No need to duplicate them externally.

[How]
Remove redundant safeguards.
Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1329,10 +1329,7 @@ static void amdgpu_dm_fini(struct amdgpu_device *adev)
 	}
 #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;
-	}
+	dc_dmub_srv_destroy(&adev->dm.dc->ctx->dmub_srv);
 
 	if (dc_enable_dmub_notifications(adev->dm.dc)) {
 		kfree(adev->dm.dmub_notify);