drm/radeon/cik: use a mutex to properly lock srbm instanced registers

We need proper locking in the driver when accessing instanced
registers on CIK.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/radeon/radeon.h b/drivers/gpu/drm/radeon/radeon.h
index 76dbe8e..274b8e1 100644
--- a/drivers/gpu/drm/radeon/radeon.h
+++ b/drivers/gpu/drm/radeon/radeon.h
@@ -2095,6 +2095,8 @@
 	/* ACPI interface */
 	struct radeon_atif		atif;
 	struct radeon_atcs		atcs;
+	/* srbm instance registers */
+	struct mutex			srbm_mutex;
 };
 
 int radeon_device_init(struct radeon_device *rdev,