drm/radeon: add DisplayPort MST support (v2)

This adds initial DP 1.2 MST support to radeon, on CAYMAN
and up in theory.

This is off by default.

v2: agd5f:
- add UNIPHY3 offsets
- move atom cmd table code into atombios_encoders.c
- whitespace cleanup
- replace some magic numbers with proper defines

Signed-off-by: Dave Airlie <airlied@redhat.com>
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 1506332..35ab65d 100644
--- a/drivers/gpu/drm/radeon/radeon.h
+++ b/drivers/gpu/drm/radeon/radeon.h
@@ -112,6 +112,7 @@
 extern int radeon_bapm;
 extern int radeon_backlight;
 extern int radeon_auxch;
+extern int radeon_mst;
 
 /*
  * Copy from radeon_drv.h so we don't have to include both and have conflicting