Reflect the status of obj->mapping as added with the
i915_gem_object_pin_map API.

'M' was chosen to designate the pin mapped status.
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>