drm/i915/selftests: Impose a timeout for request submission

Avoid spinning indefinitely waiting for the request to be submitted, and
instead apply a timeout. A secondary benefit is that the error message
will show which suspect is blocked.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106114234.2529613-2-chris@chris-wilson.co.uk