Revert "minigbm: i915: libdrm-ize backend"

Revert "minigbm: i915: libdrm-ize backend"

As per offline discussion, libdrm_intel doesn't maintain cache coherency
correctly for a number of edge cases.

The plan is to:

1) Revert the libdrm CL.
2) Use simpler mmap/unmap logic to deal CTS failures.
3) Use explicit flush to deal with coherency.

CQ-DEPEND=CL:454105

This reverts commit 82a8eedaa5d01dea4c244be751e950566dcd274e.
Conflicts: i915.c

Change-Id: I6c0bf746b71ea08f5cf4f1aa80d17d28b26ec4c6
Reviewed-on: https://chromium-review.googlesource.com/454104
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Dominik Behr <dbehr@chromium.org>