iris: Get VM id from iris_bufmgr's fd

iris: Get VM id from iris_bufmgr's fd

iris_bufmgr_init_global_vm() was getting the VM id from the callers
fd not the iris_bufmgr's duplicated fd.

If i915 driver decides to always return unique vm ids, like a counter
that is initialized per device. So fd0 would have vm id = 0 by
default, fd1(dup of fd0) would have vm id = 1... in this scenario it
would cause a mismatch. But it is not the current i915 implementation.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19467>