gallium/swr: Cleaned up some context-resource management

gallium/swr: Cleaned up some context-resource management

Removed bound_to_context.  We now pick up the context from the screen
instead of the resource itself.  The resource could be out-of-date
and point to a pipe that is already freed.

Fixes manywin mesa xdemo.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>