msm_fb: display: avoid staging up two pipes at same mixer_stage

staging two pipes at same mixer stage can cause blue screen, adding
additional check to avoid this.

Change-Id: Iff116f819236f8f3fe44635dea94cce2ba60b766
CRs-Fixed: 304278
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>