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

Re-enable GFX PG.  It's working properly with MEC now that KIQ is
enabled.

Reviewed-by: Samuel Li <samuel.li@amd.com>

This reverts commit e9ef19aa1bdeac380662a112f1d03a7c3477527f.