i965: Deferred allocation of mcs for lossless compressed

i965: Deferred allocation of mcs for lossless compressed

Until now mcs was associated to single sampled buffers only for
fast clear purposes and it was therefore the responsibility of the
clear logic to allocate the aux buffer when needed. Now that normal
3D render or blorp blit may render with mcs enabled also, they need
to prepare the mcs just as well.

v2: Do not enable for scanout buffers
v3 (Ben):
   - Fix typo in commit message.
   - Check for gen < 9 and return early in brw_predraw_set_aux_buffers()
   - Check for gen < 9 and return early in intel_miptree_prepare_mcs()
v4: Check for msaa_layput and number of samples to determine if
    lossless compression is to used. Otherwise one cannot distuingish
    between fast clear with and without compression.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>