tools headers UAPI: Update tools's copy of drm.h header

Picking the changes from:

  43d5ac7d ("drm: document DRM_IOCTL_MODE_GETFB2")

It is just a comment, so no changes and silences these perf build warnings:

  Warning: Kernel ABI header at 'tools/include/uapi/drm/drm.h' differs from latest version at 'include/uapi/drm/drm.h'
  diff -u tools/include/uapi/drm/drm.h include/uapi/drm/drm.h

Cc: Simon Ser <contact@emersion.fr>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>