drm/amdgpu: Enable SDMA utilization for Arcturus

SDMA utilization calculations are enabled/disabled by
writing to SDMAx_PUB_DUMMY_REG2 register. Currently,
enable this only for Arcturus.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>