drm/msm/shd: fix null atomic state during mode_fixup

Add base display into atomic state to fix the null atomic state
issue during bridge's mode_fixup. Also simplify base display setup
to avoid manual commit.

Change-Id: Ie6df831735d553819ab4327259ab4674c30d9bf6
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>