drm/amdgpu: Set correct DMA mask for aldebaran

Aldebaran has 48-bit physical address support

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>