iris: fix export of GEM handles

iris: fix export of GEM handles

We reuse DRM file descriptors internally. Therefore when we export a
GEM handle we must do so in the file descriptor used externally.

This change also fixes a file descriptor leak of the FD given at
screen creation.

v2: Don't bother checking fd equals, they're always different
    Fix dmabuf leak
    Fix GEM handle leaks by tracking exported handles

v3: Check os_same_file_description error (Michel)
    Don't create multiple exports for a given GEM table

v4: Add WARN_ONCE (Ken)
    Rename external_fd to winsys_fd

v5: Remove export lock in favor of bufmgr's

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2882
Fixes: 7557f1605968 ("iris: share buffer managers accross screens")
Tested-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4861>