drm/msm/sde: fix pp-split boot up with continuous splash enabled

Current driver does not boot up properly when device
boots with continuous splash enabled and pp-split
configuration. Fix this problem by adding the
correct checks that were causing a null pointer
dereference and a wrong configuration.

Change-Id: I2b6b499a38305e5bd98a52e8ff606bd843dd0199
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>