radeon/kms: fix dma relocation checking

We were checking the index against the size of the relocation buffer
instead of against the last index. This fix kernel segfault when
userspace submit ill formated command stream/relocation buffer pair.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>