docs: Use :internal: for include/drm/drm_syncobj.h

Documentation/gpu/drm-mm.rst includes from include/drm/drm_syncobj.h with
:export:, but this is a header file without export directives.  That
results in this warning:

    ./include/drm/drm_syncobj.h:1: warning: no structured comments found

...and a failure to obtain the documentation from that file.  Switch to
:internal: instead to make both problems go away.

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>