msm: mdss: Clear PP software state when fb device is released

The postprocessing software state retaining which features are enabled
needs to be cleared when an fb device is released. Otherwise, the PP
features will be reenabled the next time a fb device is opened, writing
stale configurations to the hardware which are no longer valid.

Change-Id: Ie02670b731a8a89fb578c16d545af5daa46f8c2e
Signed-off-by: Benet Clark <benetc@codeaurora.org>