radeonsi: fix an assertion failure: assert(!res->b.is_shared)

This only appears to happen on Raven2.

Possible way to reproduce:

resource_get_handle(WINSYS_HANDLE_TYPE_KMS) --> sets is_shared = true
resource_get_handle(WINSYS_HANDLE_TYPE_DMABUF) --> fail

Cc: 19.1 19.2 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 8d0d753b)