msm: mdss: Allocating pipe even cleanup map is Null

There is synchronisation issue between overlay prepare and cleanup
pipe on different frame buffers. When prepare is called on one frame buffer
and clean up of same pipe is called from other frame buffer, there are chances
where pipe is destroyed between pipe setup and wait4pipe func, because of
which cleanup map shows null and returns with out calling mdss_mdp_pipe_alloc.
To fix the issue always try to allocate pipe even when cleanup map is NULL.

Change-Id: I2abeebaef28c4937f91b7ff40cb8e0a38d1de3d4
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>