drm/msm/dpu: add layer mixer register dump to dpu snapshot

Add the missing layer mixer register dump information to
dpu snapshot to assist debugging.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Link: https://lore.kernel.org/r/1639621658-1500-1-git-send-email-quic_abhinavk@quicinc.com


Signed-off-by: Rob Clark <robdclark@chromium.org>