svga: cache the backing surface handle in the texture object

svga: cache the backing surface handle in the texture object

CinebenchR15 not only binds the same texture for rendering and sampling,
it actually changes the framebuffer buffer attachment very often, causing
a lot of backed surface view to be created and a lot of surface copies
to be done. This patch caches the backed surface handle
in the texture resource and allows the backed surface view to
reuse the backed surface handle.  With this patch, the number of
backed surface view reduces from 1312 to 3. Unfortunately, this
does not eliminate all the surface copies. There are still surface
copies involved when we switch from original to backed surface handle
for rendering.

Tested with CinebenchR15, NobelClinicianViewer, Turbine, Lightsmark2008,
            MTT glretrace, MTT piglit.

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