drm/i915: allow cache sharing policy control

Expose the SNB+ cache sharing policy register in debugfs.  The new file,
i915_cache_sharing, has 4 values, 0-3, with 0 being "max uncore
resources" and 3 being the minimum.  Exposing this control should make
benchmarking easier and help us choose a good default.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>