i965: Consolidate some of the intel_blit logic

i965: Consolidate some of the intel_blit logic

An upcoming patch is going to introduce some code here, and having this code
organized as the patch does makes it a bit easier to read later.

There should be no functional change here.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>