radv: Don't segfault when exporting an image which hasn't been bound yet.

radv: Don't segfault when exporting an image which hasn't been bound yet.

The image is set on Memory allocation already, but the image doesn't
have to have the BindImageMemory called yet. Luckily, we know offset
within a BO has to be 0 for dedicated allocations, so we can just
use the dummy 0 in the address calaculations.

Fixes CTS test dEQP-VK.api.external.memory.opaque_fd.dedicated.image.export_bind_import_bind

Signed-off-by: Bas Nieuwenhuizen <basni@google.com>
Fixes: b70829708ac "radv: Implement VK_KHR_external_memory"
Reviewed-by: Dave Airlie <airlied@redhat.com>