svga: don't reference count svga_sampler_view's texture

svga: don't reference count svga_sampler_view's texture

svga_sampler_view contains a pointer to a pipe_resource (base class of
svga_texture) and svga_texture contains a pointer to an svga_sampler_view.
This circular dependency prevented the objects from ever being freed when
they pointed to each other.  Make the svga_sampler_view::texture pointer
a "weak reference" (no reference counting) to break the dependency.

This is safe to do because the pipe_resource/texture always has a longer
lifespan than the sampler view so when svga_sampler_view stops referencing
the texture, the texture's refcount never hits zero.

Fixes a memory leak seen with google earth and other apps.

Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>