minigbm: add buffer invalidation function

minigbm: add buffer invalidation function

We may need to invalidate a buffer before reading it. Some use
cases are:

	- DRM_IOCTL_I915_GEM_SET_DOMAIN
        - DMA_BUF_IOCTL_SYNC with the SYNC_START option
        - DRM_IOCTL_VIRTGPU_TRANSFER_FROM_HOST

This patch adds the function hook.

BUG=chromium:764871
TEST=compiles

Change-Id: I85811407252b859a12294381c65ff3545424636b
Reviewed-on: https://chromium-review.googlesource.com/710322
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>