This implements PIPE_CAP_INVALIDATE_BUFFER and invalidate_resource(),
as well as the PIPE_TRANSFER_DISCARD_WHOLE_RESOURCE flag.  When either
of these happen, we swap out the backing storage of the buffer for a
new idle BO, allowing us to write to it immediately without stalling
or queueing a blit.

On my Skylake GT4e at 1920x1080, this improves performance in games:

   -----------------------------------------------
   | DiRT Rally        | +25% (avg) | +17% (max) |
   | Bioshock Infinite | +22% (avg) | +11% (max) |
   | Shadow of Mordor  | +27% (avg) | +83% (max) |
   -----------------------------------------------