drm/amdgpu: fix a GPU hang issue when remove device

When GFXOFF is enabled and GPU is idle, driver will fail to access some
registers. Therefore change to disable power gating before all access
registers with MMIO.

Dmesg log is as following:
amdgpu 0000:03:00.0: amdgpu: amdgpu: finishing device.
amdgpu: cp queue pipe 4 queue 0 preemption failed
amdgpu 0000:03:00.0: amdgpu: failed to write reg 2890 wait reg 28a2
amdgpu 0000:03:00.0: amdgpu: failed to write reg 1a6f4 wait reg 1a706
amdgpu 0000:03:00.0: amdgpu: failed to write reg 2890 wait reg 28a2
amdgpu 0000:03:00.0: amdgpu: failed to write reg 1a6f4 wait reg 1a706

Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>