msm: mdss: Fix error value on pipe priority mismatch

MDP has a HW limitation that when source split is enabled
and two pipes are staged on the same zorder of a mixer,
the left pipes must of be higher priority than the right.
If MDP couldn't find a right pipe of lower priority, instead
of returning EINVAL, it should return NULL indicating its
failure to allocate a pipe.

Change-Id: Ife9dff64bb17f56b22f9a4015f621c70d7e6c735
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>