intel: Align untiled buffer pitch to 64B.

This is the largest untiled pitch requirement from gen2 through gen4.
It's only the case for gen3 rendering to color regions with depth, but
it's rare for this to be a significant factor in memory usage -- for
example, gen4 requires 1 or 2 times the element size, or up to 64
bytes depending on the size of the elements.  This is easier than
encoding all the various little quirks for untiled pitch alignment,
since we rarely do untiled now.