mdss: fb: wait for idle only on last fb release

There could be multiple references to the framebuffer, when
releasing the last one the panel should be blanked. When blanking the
panel we should make sure that panel update has finished (wait for panel
idle), however this wait should be done only if it will lead to a
change in the panel. Avoid this wait if there are more than one ref
counts which means that panel won't be blanked.

Change-Id: I22ffd25cfc6d0512d006a979613610314772a27e
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>