svga: fix texture upload path condition

svga: fix texture upload path condition

As Thomas suggested, we'll first try to map directly to a GB surface.
If it is blocked, then we'll use texture upload buffer.
Also if a texture is already "rendered to", that is, the GB surface
is already out of sync, then we'll use the texture upload buffer
to avoid syncing the GB surface.

Tested with Lightsmark2008, Tropics, MTT piglit, glretrace.

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