msm_fb: display: fix page fault during supend/resume

There has VG1 pipe commit (pan display) happen after system
suspended. This left VG1 still staged up at mdp mixer. Once timing
generator is enabled at resume, VG1 pipe start fetching contents from
address 0 since VG1 has not yet be configured. This cause page fault.
This patch has sanity check at system suspend to make sure no any pipe
stage up at mixer after suspend.

Change-Id: Idcf974ceb4afe2a3ec55b9603b700fa497f84045
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>