drm/amdgpu: use correct register mask to extract field

Aldebaran has different register mask definitions for
regiter MC_VM_XGMI_LFB_CNTL. Use the correct masks
to interpret fields of this register.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>