mesa/swrast: use logf2 instead of util_fast_log2

The fast version is apparently not accurate enough. I wrote a very simply test program that called logf2 and the old LOG2 function 100000 times. Across that the two functions had very similar run times, neither appeared meaningfully faster, so the optimization of bringing back yet another way to calculate log2f seems pointless.

Fixes: bd4e7695 ("replace LOG2 with util_fast_log2") Closes: #2856 (closed)