iris: Track imported vs. exported status separately

iris: Track imported vs. exported status separately

Not all external objects are the same.  Imported buffers may be from
other devices (say a dmabuf from an AMD or NVIDIA discrete card) which
are backed by memory that we can't use with I915_GEM_MMAP.  However,
exported buffers are ones that we know we allocated ourselves from our
own device.  We may not know what other clients are doing with them,
but we can assume a bit more about where they came from.

Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10941>