libdrm: NOTE! Default branch is now main

To update your local repository to use the new default branch, these
commands may help:

$ git fetch origin
$ git checkout master
$ git branch -m main
$ git branch --set-upstream-to=origin/main
$ git remote set-head origin --auto

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>