drm/amdgpu/gfx8: Switch Stoney to share CZ's RLC functions

According to the bringup code ST/CZ share the RLC
ENTER/EXIT logic.

Tested on my ST board.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>