drm/msm/dpu: drop src_split and multirect check from dpu_crtc_atomic_check

Neither source split nor multirect are properly supported at this
moment. Both of these checks depend on normalized_zpos being equal for
several planes (which is never the case for normalized zpos).
Drop these checks to simplify dpu_crtc_atomic_check(). The actual
support for either of these features is not removed from the backend
code (sspp, ctl, etc).

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>