drm/msm/sde: Use primary display for splash

In current implementation, splash uses single display
path, so for allocating resources for splash, ensure
primary display is used as otherwise there are chances
of mixer allocation failures.

Change-Id: I63554fce4b39fe1dc94d0648eb69a4007616378d
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>