drm: Make syncobj import/export functions static

Fix the following sparse warnings by making functions static:
drm_syncobj.c:420:5: warning: symbol 'drm_syncobj_import_sync_file_fence' was not declared. Should it be static?
drm_syncobj.c:441:5: warning: symbol 'drm_syncobj_export_sync_file' was not declared. Should it be static?

Cc: Dave Airlie <airlied@redhat.com>
Fixes: 3ee45a3b

 ("drm/syncobj: add sync_file interaction. (v1.2)")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901165328.24459-1-ville.syrjala@linux.intel.com
Reviewed-by: Thierry Reding <treding@nvidia.com>