drm/i915: Check engine->default_state mapping on module load

Check we can indeed acquire a WB mapping of the context image on module
load. Later this will give us the opportunity to validate that we can
switch from WC to WB as required.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180914123504.2062-2-chris@chris-wilson.co.uk