minigbm: virtgpu: add virgl_protocol.h, add external directory

minigbm: virtgpu: add virgl_protocol.h, add external directory

This adds Mesa protocol headers for BLOB resources.  Also some
code movement.  See:

https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4821

for another example.

BUG=chromium:924405
TEST=compile

Change-Id: I330851e6a58d4e12be8e0e882a230a1cca83e7a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/1804828
Reviewed-by: Jason Macnak <natsu@google.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Commit-Queue: Gurchetan Singh <gurchetansingh@chromium.org>