drm/msm/dpu: drop the `smart_dma_priority' field from struct dpu_sspp_sub_blks

In preparation to deduplicating SSPP subblocks, drop the (unused)
`smart_dma_priority' field from struct dpu_sspp_sub_blks. If it is
needed later (e.g. for SmartDMA v1), it should be added to the SSPP
declarations themselves.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/570103/
Link: https://lore.kernel.org/r/20231201234234.2065610-6-dmitry.baryshkov@linaro.org