drm/msm/mdp4: pageflip fixes

Backport a few fixes found in the course of getting mdp5 working.
There is a window of time after pageflip is requested, before we
start scanning out the new fb (ie. while we are waiting for gpu).
During that time we need to continue holding a reference to the
still-current scanout fb, to avoid the backing gem bo's from being
destroyed.

Possibly a common mdp_crtc parent class could be useful to share
some of this logic between mdp4_crtc and mdp5_crtc.  OTOH, this
all can be removed from the driver once atomic is in place, as
plane/crtc updates get deferred until all fb's are ready before
calling in to .page_flip(), etc.

Signed-off-by: Rob Clark <robdclark@gmail.com>