drm/radeon: move drm_buffer to drm/radeon/

Radeon UMS is the last user of drm_buffer. Move it out of sight so radeon
can drop it together with UMS.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>