drm/amdgpu/gfx9: Raven has two MECs

This was missed when Andres' queue patches were rebased.

Fixes: 42794b27

 (drm/amdgpu: take ownership of per-pipe configuration v3)
Reviewed-by: Alex Xie <AlexBin.Xie@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>