drm/fb-helper: Push locking in fb_is_bound

That function only needs to take the individual crtc locks, not all
the kms locks. Push down the locking and then minimize it.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Thierry Reding <treding@nvidia.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170704151833.17304-5-daniel.vetter@ffwll.ch