swr/rast: bypass size limit for non-sampled textures

This fixes a bug where SWR will fail to render in cases with large
buffer allocations, e.g. very large meshes whose vertex buffers exceed
2GB

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>
(cherry picked from commit 6053499f)