drm/amd/amdgpu: fw version check with gfxoff

1. check the firmware version when enabling gfxoff
2. overwrite the pptable to make sure gfxoff is really
enabled on navi10

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>