i965: Don't try to create aux buffer for non-msrt aux-buffer

i965: Don't try to create aux buffer for non-msrt aux-buffer

In addition to simply calling miptree_create() the higher level
call intel_miptree_create() also considers if the buffer should
be associated with an auxiliary buffer based on the given format.

Here we are allocating an auxiliary buffer which in turn has such
format that would mislead intel_miptree_create_layout() later on
to try to associate the auxiliary buffer with an auxiliary buffer.
To prevent this the actual buffer creation logic was split out
into its own function. Lets invoke that instead.

v2 (Ben): Do not signal msaa layout with explicit argument but
          using layout_flags instead.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com>