drm: msm: sde: add splash handoff support for shared display

When shared display is both enabled in bootloader and kernel,
the handoff mechanism needs to be updated to ensure the handoff
can be finished.

Change-Id: I8172b0878a4a322f87e0ad138cff1e27161a9f83
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>