virgl: do not map zero-sized resource

virgl: do not map zero-sized resource

When creating textures, we avoid creating backing-store for all
multisampled textures, not just depth buffers.

So we can't try to map them later. That's just going to fail. So
let's take the blit-based code-path that seems to avoid this problem.

This make this piglit test-case no longer crash (although it still
fails):

bin/copyteximage 2D -samples=2 -auto

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>