drm/msm/dpu: check dpu_plane_atomic_print_state() for valid sspp

Similar to the r_pipe sspp protect, add a check to protect
the pipe state prints to avoid NULL ptr dereference for cases when
the state is dumped without a corresponding atomic_check() where the
pipe->sspp is assigned.

Fixes: 31f7148f ("drm/msm/dpu: move pstate->pipe initialization to dpu_plane_atomic_check")
Reported-by: Stephen Boyd <swboyd@chromium.org>
Closes: drm/msm#67


Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Stephen Boyd <swboyd@chromium.org> # sc7180-trogdor
Patchwork: https://patchwork.freedesktop.org/patch/628404/
Link: https://lore.kernel.org/r/20241211-check-state-before-dump-v2-1-62647a501e8c@quicinc.com


Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>