nv30: disable rendering to 3D textures

There's no way to tell the 3D engine about swizzling on such textures.
While rendering to NPOT ones may be possible, there's no great way to
expose that in gallium, nor would there be any practical benefit.

Fixes the non-compressed-format "copyteximage 3D" failures. Something
odd going on with the compressed formats.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/nv30/nv30_screen.c

    if (MAX2(1, sample_count) != MAX2(1, storage_sample_count))
       return false;
 
+   /* No way to render to a swizzled 3d texture. We don't necessarily know if
+    * it's swizzled or not here, but we have to assume anyways.
+    */
+   if (target == PIPE_TEXTURE_3D && (bindings & PIPE_BIND_RENDER_TARGET))
+      return false;
+
    /* shared is always supported */
    bindings &= ~PIPE_BIND_SHARED;