drm/msm/sde: Add SDM855 support in REG DMA framework

REG DMA functionality has be expanded on SDM855, this change
adds the corresponding support in the existing REG DMA framework.

Change-Id: If125c0696e8953f90eec823c8812193031e211c8
Signed-off-by: Ping Li <pingli@codeaurora.org>