llvmpipe: avoid flushing depth buffer cache on swapbuffers

llvmpipe: avoid flushing depth buffer cache on swapbuffers

There's no need to push out depth buffer contents on swapbuffers.

Note that this change doesn't throw away depth buffer changes, it simply
holds them in the cache over calls to swapbuffers.  The hope is
that swapbuffers will be followed by a clear() which means in that case
we won't have to write the changes out.