drm/ast: Fix double lock at PM resume

The recent commit [3ea87855: drm/helper: lock all around force mode
restore] introduced drm_modeset_lock_all() in
drm_helper_resume_force_mode() itself, while ast driver still takes
this lock before calling it.  Remove the caller side lock for avoid a
fatal deadlock.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>