drm/msm/sde: Initialize REG_DMA sspp_buf for all the rects

For multirect use cases, REG_DMA needs to initialize sspp_buf
for all rects, including rect_solo, rect_0 and rect_1. This change
adds the missing rect_solo case.

Change-Id: Ic04881eb4dcfcb5621d30c9dea8a068c61817010
Signed-off-by: Ping Li <pingli@codeaurora.org>