drm/i915/selftests: Try to detect rollback during batchbuffer preemption

Since batch buffers dominant execution time, most preemption requests
should naturally occur during execution of a batch buffer. We wish to
verify that should a preemption occur within a batch buffer, when we
come to restart that batch buffer, it occurs at the interrupted
instruction and most importantly does not rollback to an earlier point.

v2: Do not clear the GPR at the start of the batch, but rely on them
being clear for new contexts.

Suggested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200422100903.25216-1-chris@chris-wilson.co.uk