msm: mdss: initialize pipe to NULL

Initialize the pipe pointer to NULL.
So that even if the selection of pipe from pipe-pool
fails, the function returns NULL instead of an
uninitialized value.

Change-Id: Ib04c0a983cba6ffe9532862bcaf516877e49746a
CRs-Fixed: 524466
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>