panfrost: Get rid of the "free imported BO" logic

panfrost: Get rid of the "free imported BO" logic

bo->imported was never set to true which means this path was never taken.
Moreover, panfrost_drm_free_imported_bo() is doing missing the munmap()
call which seems wrong because the import BO function calls mmap().

Let's just kill this function along with the ->imported field.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>