i965: Skip reallocating the private MSAA miptree, unless it's resized.

i965: Skip reallocating the private MSAA miptree, unless it's resized.

Even if the singlesample_mt got reopened from DRI due to
pageflipping/buffer swapping, our private miptree shouldn't need any
changes.

Improves performance of a little swapbuffers-loving microbenchmark with
MSAA forced on, by 1.2371% +/- 0.624802% (n=102)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>