drm/msm/dpu: drop redundant plane dst check from dpu_crtc_atomic_check()

The helper drm_atomic_helper_check_plane_state() already checks whether
the scaled and clipped plane falls into the CRTC visible region (and
clears plane_state->visible if it doesn't). Drop the redundant check
from dpu_crtc_atomic_check().

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>