Attempt to stress test performing relocations whilst the batch is in the
CPU domain.

A freshly allocated buffer starts in the CPU domain, and the pwrite
should also be performed whilst in the CPU domain and so we should
execute the relocations within the CPU domain. If for any reason one of
those steps should land it in the GTT domain, we take the secondary
precaution of filling the mappable portion of the GATT.

In order to detect whether a relocation fails, we first fill a target
buffer with a sequence of invalid commands that would cause the GPU to
immediate hang, and then attempt to overwrite them with a legal, if
short, batchbuffer using a BLT. Then we come to execute the bo, if the
relocation fail and we either copy across all zeros or garbage, then the
GPU will hang.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>