drm/msm: remove unbalanced mutex unlock

This regression stems from 0e08270a ("drm/msm: Separate locking of
buffer resources from struct_mutex").

Signed-off-by: Daniel Mack <daniel@zonque.org>
Cc: Sushmita Susheelendra <ssusheel@codeaurora.org>
Cc: Rob Clark <rclark@redhat.com>
Fixes: 0e08270a ("drm/msm: Separate locking of buffer resources from struct_mutex")
Signed-off-by: Rob Clark <robdclark@gmail.com>