minigbm: i915: use explicit cache flush to maintain coherency

minigbm: i915: use explicit cache flush to maintain coherency

The Intel last level cache (LLC) acts a coherency controller on
hardware that has it. Use I915_PARAM_HAS_LLC to determine if the
hardware has LLC.

We can maintain coherency by doing a clflush on non-LLC platforms, like the
Vulkan driver in Mesa does.

BUG=none
TEST=gbmtest on Cyan, Chrome boots, Android boots, mmap_test -g, 5-10 CTS
tests. No regressions.

CQ-DEPEND=CL:425716

Change-Id: I054e69c23016cd3336c2de1d07b49bb2c3709389
Reviewed-on: https://chromium-review.googlesource.com/454105
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>