drm/helper: lock all around force mode restore

Since Daniel documented things with a sledge hammer, we got lots of
nice backtraces in suspend/resume operations, I've check the callers
of this and they all seems safe to me,

This fixes one set of warns I reported.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>