drm/amdgpu: move adjust_mc_addr into amdgpu_gart_funcs

We should probably rename amdgpu_gart_funcs sooner or later.

Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>