drm/msm/dpu: don't use unsupported blend stages

drm/msm/dpu: don't use unsupported blend stages

The dpu_crtc_atomic_check() compares blending stage with DPU_STAGE_MAX
(maximum amount of blending stages supported by the driver), however we
should compare it against .max_mixer_blendstages, the maximum blend
stage supported by the mixer.

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/527338/
Link: https://lore.kernel.org/r/20230316161653.4106395-16-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>