msm_fb: display: free pipe if blt is detected for the pipe

Once blt is detected, overlay set will return fail to disable blt
mode, but the pipe was already allocated, so it is necessary to free
the pipe before overlay set returns to userspace.

Change-Id: Id9352c65a890b594ea15c85571233090c1f7d323
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>