intel: Improve bo_references performance by skipping the tree walk.

If the target we're asking about hasn't ever been used as a relocation
target, then it obviously hasn't been used as a target by the batch's reloc
tree.  This is the common case for good GL programming where you only map
fresh buffers, and gives us a 5% win in cairo-gl.

[ # ]  backend                         test   min(s) median(s) stddev. count
before:
[  0]       gl            firefox-talos-gfx   64.680   64.756   0.06%    3/3
after:
[  0]       gl            firefox-talos-gfx   60.816   60.970   0.29%    3/3