drm/msm/sde: add plane check in continuous splash case

In dual display continuous splash case, there are certain
scenarios where pipe being used in secondary display at boot up
is allocated by primary crtc. Add check to return failure
in such cases.

Change-Id: I9047b6e7f91e59a9daff5089abb41017c068b449
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>