intel: Only call clock_gettime once per unreference_final.

Notably when freeing a batchbuffer, we often end up freeing many of the
buffers it points at as well.  Avoiding repeated calls brings us a 9% CPU
win for cairo-gl.

[ # ]  backend                         test   min(s) median(s) stddev. count
before:
[  0]       gl            firefox-talos-gfx   58.941   58.966   0.75%    3/3
after:
[  0]       gl            firefox-talos-gfx   54.186   54.195   0.49%    3/3