msm: mdss: fix timing gen flush sequence for dynamic fps

Timing generator update cannot happen in the
same frame boundary/vsync cycle where a change
in the mixer configuration is requested.
Enforce this sequence by moving timing generator
programming to the commit thread, so driver can
wait for one vsync cycle when timing generator
has been modified due a dynamic fps update.

Change-Id: Ifac5332f3de45479ebc0a6ab0c1f7cf7b48a41b0
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>