r600: don't do stack workarounds for hemlock

By the looks of it it seems hemlock is treated separately to cypress, but
certainly it won't need the stack workarounds cedar/redwood (and
seemingly every other eg chip except cypress/juniper) need.
(Discovered by accident.)

Acked-by: Alex Deucher <alexander.deucher@amd.com>

src/gallium/drivers/r600/sb/sb_bc.h

@@ -665,6 +665,7 @@ public:
 			return false;
 
 		switch (hw_chip) {
+		case HW_CHIP_HEMLOCK:
 		case HW_CHIP_CYPRESS:
 		case HW_CHIP_JUNIPER:
 			return false;