drm/amd/amdgpu: Disable GFX_PG on Carrizo until compute issues solved

Currently compute jobs will stall if GFX_PG is enabled.  Until this
is resolved we'll disable GFX_PG.

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