drm/i915: Mock infrastructure for request emission

Create a fake engine that runs requests using a timer to simulate hw.

v2: Prevent leaks of ctx->name along error paths

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-8-chris@chris-wilson.co.uk