svga: Fix banded DMA upload unmap

Even with banded DMA uploads, st->hwbuf is always non-NULL, but when we've
allocated a software buffer to hold the full upload, unmapping of the
hardware buffer has already been done before
svga_texture_transfer_unmap_dma(), and the code was performing an unmap of
an already mapped buffer.

Fix this by testing for software buffer not present.

Fixes: a9c4a861 ("svga: refactor svga_texture_transfer_map/unmap functions")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
(cherry picked from commit 00db9769)