drm/amdgpu/gfx: set cg flags to enter/exit safe mode

sriov needs to enter/exit safe mode in update umd p state
add the cg flag to let it enter or exit while needed

Signed-off-by: Jane Jian <Jane.Jian@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>