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On SOC-15 parts, the GMC (Graphics Memory Controller) consists
of two hubs: GFX (graphics and compute) and MM (sdma, uvd, vce).

v2: drop sdma from Makefile, fix duplicate return statement.
Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>