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.

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