Add support to set dynamic fps

* This change allows fps to be changed dynamically
at run time. The property "persist.hwc.dynamicfps"
can be used to set the fps.

* fps can only be set once the ctrl path is setup.
This is achieved by ensuring that display commits
until and including the first non-blank commit
are synchronous and fps is set after that.

Change-Id: Idf31ed2974320ca22637097bcf72b58471a030b7