drm/amdgpu/gfx8: Enable GFX PG on CZ

Based on Alex's patches this enables GFX PG on CZ.

Tested with xonotic-glx/glxgears/supertuxkart and idle desktop.
Also read-back registers via umr for verificiation that the bits
are truly enabled.

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>