drm/amdgpu: ungate power gating when system suspend

[Why] During suspend, if GFX DPM is enabled and GFXOFF feature is
enabled the system may get hung. So, it is suggested to disable
GFXOFF feature during suspend and enable it after resume.

[How] Update the code to disable GFXOFF feature during suspend and enable
it after resume.

[  311.396526] amdgpu 0000:03:00.0: amdgpu: SMU: I'm not done with your previous command: SMN_C2PMSG_66:0x0000001E SMN_C2PMSG_82:0x00000000
[  311.396530] amdgpu 0000:03:00.0: amdgpu: Fail to disable dpm features!
[  311.396531] [drm:amdgpu_device_ip_suspend_phase2 [amdgpu]] *ERROR* suspend of IP block <smu> failed -62

Acked-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Kun Liu <kun.liu2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org