drm/msm/dpu: get rid of DPU_CTL_DSPP_SUB_BLOCK_FLUSH

drm/msm/dpu: get rid of DPU_CTL_DSPP_SUB_BLOCK_FLUSH

Continue migration to the MDSS-revision based checks and replace
DPU_CTL_DSPP_SUB_BLOCK_FLUSH feature bit with the core_major_ver >= 7
check.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/655380/
Link: https://lore.kernel.org/r/20250522-dpu-drop-features-v5-11-3b2085a07884@oss.qualcomm.com