drm/radeon/si: disable cgcg and pg for now

Coarse grain clockgating causes problems with reclocking on
some cards and powergating (verde only) causes problems with
ring initialization.  The proper fix (restructuring the init
sequences) is too invasive for 3.11 so just disable them for
now.

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