i965/blorp: Refactor do_single_blorp_clear

i965/blorp: Refactor do_single_blorp_clear

Previously, we had two checks for can_fast_clear and a tiny bit of
shared code in between.  This commit pulls all of the fast clear code
together and duplicates the tiny bit that declares some surface structs
and calls blorp_surf_for_miptree.  The duplication is no real loss and
we're about to change the two in slightly different ways.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>