radeonsi: fix the R600_RESOURCE_FLAG_UNMAPPABLE check

The flag is on the pipe_resource, not the r600_resource.

I don't see an obvious bug related to this, but it could potentially lead
to suboptimal placement of some resources.

Fixes: a4158743 ("gallium/radeon: add R600_RESOURCE_FLAG_UNMAPPABLE")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
(cherry picked from commit 5e2962c9)