svga: stop using pipe_sampler_view_release()

svga: stop using pipe_sampler_view_release()

This function was used in the past to avoid deleting a sampler view
for a context that no longer exists.  But the Mesa state tracker
ensures that cannot happen.  Use the standard refcounting function
instead.

Also, remove the code which checked for context mis-matches in
svga_sampler_view_destroy().  It's no longer needed since implementing
the zombie sampler view code in the state tracker.

Testing Done: google chrome, variety of GL demos/games

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Neha Bhende <bhenden@vmware.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-By: Jose Fonseca <jfonseca@vmware.com>