drm/msm/dpu: split pipe handling from _dpu_crtc_blend_setup_mixer

drm/msm/dpu: split pipe handling from _dpu_crtc_blend_setup_mixer

Rework _dpu_crtc_blend_setup_mixer() to split away pipe handling to a
separate functon. This is a preparation for the r_pipe support.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Tested-by: Abhinav Kumar <quic_abhinavk@quicinc.com> # sc7280
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/527354/
Link: https://lore.kernel.org/r/20230316161653.4106395-27-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>