drm_hwcomposer: update README for clang-format-diff-11 and touch up formatting

The README.md is out of date -- clang-format-diff-5.0 is not what runs on
CI anymore. To help contributors better test their changes locally before
submitting a merge request, update to the actual linting command being run
in CI.

Signed-off-by: Benjamin Li <benl@squareup.com>
4 jobs for benl/reject-unsupported-rotations in 6 minutes and 11 seconds (queued for 1 second)