drm/amdgpu: save and restore gc hub regs

Save and restore gfxhub regs as they will be reset during mode 2

Signed-off-by: Victor Zhao <Victor.Zhao@amd.com>
Acked-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>