drm/panfrost: Consider dma-buf imported objects as resident

A GEM object constructed from a dma-buf imported sgtable should be regarded
as being memory resident, because the dma-buf API mandates backing storage
to be allocated when attachment succeeds.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Fixes: 9ccdac7a ("drm/panfrost: Add fdinfo support for memory stats")
Reported-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231125205438.375407-2-adrian.larumbe@collabora.com