drm/msm/dpu: correct MERGE_3D length

drm/msm/dpu: correct MERGE_3D length

Each MERGE_3D block has just two registers. Correct the block length
accordingly.

Fixes: 4369c93cf36b ("drm/msm/dpu: initial support for merge3D hardware block")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/542177/
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Link: https://lore.kernel.org/r/20230613001004.3426676-3-dmitry.baryshkov@linaro.org