intel: Track whether a buffer is idle to avoid trips to the kernel.

I've seen a number of apps spending unreasonable amounts of time in
drm_intel_bo_busy during the buffer mapping process.

We can't track idleness in general, in the case of buffers shared
across processes.  But this should significantly reduce our overhead
for checking for busy on things like VBOs.

Improves (unoptimized) glamor x11perf -f8text by 0.243334% +/-
0.161498% (n=1549), which has formerly been spending about .5% of its
time hitting the kernel for drm_intel_gem_bo_busy().

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