msm: mdss: Fix potential pipe NULL pointer dereference

NULL pointer derefernce can happen with variable pipe
struct during pipe intialization. Fix this to add a
check for NULL pointer.

Change-Id: I3241ed89979deb777ca62d0c893afb96926820ee
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>