msm_fb: display: fix underrun when conecting wifi display

When connecting wifi display underrun can happen if there is no frame
update in other mixers before pipe in mixer2 is staged up. Add logic to
make sure that any pipes that are going to be staged up to mixer2 are
staged down from other mixers first.

Change-Id: I84ce8d0fcd392639d470ba4161be36212bcefce1
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>