drm/radeon: expose render backend mask to the userspace

This will allow userspace to correctly program the PA_SC_RASTER_CONFIG
register, so it can be considered a fix.

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
diff --git a/drivers/gpu/drm/radeon/si.c b/drivers/gpu/drm/radeon/si.c
index 3eed9a1..85e1edf 100644
--- a/drivers/gpu/drm/radeon/si.c
+++ b/drivers/gpu/drm/radeon/si.c
@@ -2855,6 +2855,8 @@
 		mask <<= 1;
 	}
 
+	rdev->config.si.backend_enable_mask = enabled_rbs;
+
 	for (i = 0; i < se_num; i++) {
 		si_select_se_sh(rdev, i, 0xffffffff);
 		data = 0;