drm/amdgpu: fix gfx hang during suspend with video playback (v2)

The system will be hang up during S3 suspend because of SMU is pending
for GC not respose the register CP_HQD_ACTIVE access request.This issue
root cause of accessing the GC register under enter GFX CGGPG and can
be fixed by disable GFX CGPG before perform suspend.

v2: Use disable the GFX CGPG instead of RLC safe mode guard.

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Tested-by: Mengbing Wang <Mengbing.Wang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org