drm/msm/sde: register DSC_CTL and MERGE_3D blocks for debug dump

Add new hw blocks to the debug dump list so these register
values get dumped for debug analysis.  Also fix the block
length calculation during reg dump to prevent possibility
of accessing registers outside the specified max range.

Change-Id: Iee9adbe5bf0d54a2cbe7d433cc96705060353eeb
Signed-off-by: Steve Cohen <cohens@codeaurora.org>