drm/amdgpu: fix GFXOFF on Picasso and Raven2

For picasso(adev->pdev->device == 0x15d8)&raven2(adev->rev_id >= 0x8),
firmware is sufficient to support gfxoff.
In commit 98f58ada, for picasso&raven2,
return directly and cause gfxoff disabled.

Fixes: 98f58ada ("drm/amdgpu/gfx9: update pg_flags after determining if gfx off is possible")
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Aaron Liu <aaron.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org