msm_fb: display: return fail if pipe still staged

Pipe is not un-staged from mixer when it is unset.
It expects a pan_display() to un-stage it out of mixer.
Return fail at overlay_set() if pipe was still stage
at mixer.

CRs-fixed: 399193, 393979
Change-Id: I562f6e133bb239b30f4f53669dbf1cc5ddf80d02
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>