msm: mdss: rotator: implement caching of rotator acq fences

Currently rotator buf sync ioctl will replace acquire fences given by
new ones if the rotator work hasn't executed. By doing this, the
rotator work can end up waiting on fences for the next rotator work
instead of the current. To avoid this, cache the current acquire
fences so that rotator work can wait on these instead of the new ones.

Change-Id: I058fcc5e9eeb2964a227e4973d37b5f8bd08fe32
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>