drm/amdgpu/display: Unlock mutex on error

Make sure we pass through ret label to unlock the mutex.
Signed-off-by: John van der Kamp <sjonny@suffe.me.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -546,8 +546,10 @@ static ssize_t srm_data_read(struct file *filp, struct kobject *kobj, struct bin
 
 	srm = psp_get_srm(work->hdcp.config.psp.handle, &srm_version, &srm_size);
 
-	if (!srm)
-		return -EINVAL;
+	if (!srm) {
+		ret = -EINVAL;
+		goto ret;
+	}
 
 	if (pos >= srm_size)
 		ret = 0;