drm/i915: Free the imported shmemfs file for phys objects

Matthew spotted that we lost the fput() for phys objects now that we are
not relying on the core to cleanup the GEM object. (For the record, phys
objects import the shmemfs from their original set of pages and keep it
to provide swap space, but we never transform back into a shmem object.)

Reported-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 0c159ffe ("drm/i915/gem: Defer obj->base.resv fini until RCU callback")
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190809110752.19763-1-chris@chris-wilson.co.uk