drm/radeon: fix AGP userptr handling

AGP mappings are not cache coherent, so userptr support
won't work. Additional to that the AGP implementation uses
a different ttm_tt container structure so we run into
problems if we cast the pointer without checking if it's
the right type.

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