drm/i915: Mock the GEM device for self-testing

A simulacrum of drm_i915_private to let us pretend interactions with the
device.

v2: Tidy init error paths

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-6-chris@chris-wilson.co.uk