i965/blorp: Refactor fast-clear logic a bit

i965/blorp: Refactor fast-clear logic a bit

This pulls the mcs allocation into the if statement where we initially
determine that we are doing a fast clear and moves the programming of
wm_inputs and figuring out the fast clear rect into it's own if statement.
The next commit will put code inbetween the two.

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