drm/radeon/kms: dynclks fixes

- only r4xx/r5xx/rs6xx/rs740 have clock gating atom table,
so disable it on r6xx.  it's already disabled on r7xx
- check to make sure the clock_gating hook exists before
calling it.  This avoids a segfault on asics without
that function.
- remove unused static power management function.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/radeon_asic.h b/drivers/gpu/drm/radeon/radeon_asic.h
index d758b1ff..3f3c7a2 100644
--- a/drivers/gpu/drm/radeon/radeon_asic.h
+++ b/drivers/gpu/drm/radeon/radeon_asic.h
@@ -549,7 +549,7 @@
 	.set_memory_clock = &radeon_atom_set_memory_clock,
 	.get_pcie_lanes = NULL,
 	.set_pcie_lanes = NULL,
-	.set_clock_gating = &radeon_atom_set_clock_gating,
+	.set_clock_gating = NULL,
 	.set_surface_reg = r600_set_surface_reg,
 	.clear_surface_reg = r600_clear_surface_reg,
 	.bandwidth_update = &rv515_bandwidth_update,