svga: use upload buffer for upload texture.

svga: use upload buffer for upload texture.

With this patch, when running with vgpu10, instead of mapping directly to the
guest backed memory for texture update, we'll use the texture upload buffer
and use the transfer from buffer command to update the host side texture memory.

This optimization yields about 20% performance improvement with
Lightsmark2008 and about 40% with Tropics.

Tested with Lightsmark2008, Tropics, Heaven, MTT piglit, glretrace, conform.

Reviewed-by: Brian Paul <brianp@vmware.com>