drm/msm/dpu: drop length from struct dpu_hw_blk_reg_map

We (nearly) do not use the length field from struct dpu_hw_blk_reg_map,
so we can drop it safely.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/488013/
Link: https://lore.kernel.org/r/20220601161349.1517667-3-dmitry.baryshkov@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>