msm_fb: commit mixer stage for WFD during unset

Interfaces with Borderfill pipe as base layer needs to
be commited during unset which will reset the mixer cfg
structure. This is needed because overlay_commit will not
call stage commit as there is no buffer queued.

Change-Id: Ib3347e52ae437b20d5a85a09777f13aee2672edb
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>