drm/msm/shd: add dsi_display base in shd_display

Add empty dsi_display base in shd_display to avoid type-casting
error when mapping shd_display to dsi_display object.

Change-Id: I7c9997bf06a80146b30ec116f8d12a38dae725d3
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>