i965/blorp: Do prepare/finish manually

i965/blorp: Do prepare/finish manually

Our attempts to do it automatically are problematic at best.  In order
to really be precise, we need to know both the desired aux usage and
whether or not clear is supported.  The current automatic mechanism
doesn't cover this.  This commit itself is not a functional change since
it just reworks everything to be in terms of a silly helper.  Later
commits will switch things over to more sensible ways of choosing usage.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>