drm/msm/dpu: drop unused dpu_plane::lock

The field dpu_plane::lock was never used for protecting any kind of
data. Drop it now.

Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/570043/
Link: https://lore.kernel.org/r/20231201211845.1026967-8-dmitry.baryshkov@linaro.org