drm/i915: Show pin mapped status in describe_obj

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: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>