drm/amdgpu: minor cleanup for amdgpu_ttm_bind

Filter the placement mask before using it. In theory it could be that we
have other flags set here as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>