msm: mdss: reset WB mixercfg during WB destroy

Stale configs are left in layer mixer, when switching between
single/dual mixers. This might cause unexpected issues later.
Reset the configs during wfd destroy, so that it will have clean
LM configs for the next setup.

Change-Id: I3cd3699af1aa63b12e43a9d53ac505235b668e98
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>