msm_fb: display: Unstage pipe from Mixer0 H/W on each unset call for 8660

During unset call, mixer0 staging is cleared using stage down
logic which makes the staging update to happen in next vsync
for 8660 platforms. Clearing of mixer0 stage now is done on a
single vsync for 8660 as well.

Change-Id: Ib6b11728944be89ee6ba5b5d58c470c05cb9f41b
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>