i965/blorp: Refactor code for computing fast clear align/scaledown factors.

This will make it easier to add fast color clear support to MSAA
buffers, since they have different alignment and scaling requirements.
i965/gen7+: Implement fast color clears for MSAA buffers.

Fast color clears of MSAA buffers work just like fast color clears
with non-MSAA buffers, except that the alignment and scaledown
requirements are different.