WMF plugin: Initialize member variable m_stateDirty.

Fix purify warning:
[W] UMR: Uninitialized memory read in MFPlayerControl::refreshState(void) {1 occurrence}

Task-number: QTBUG-41966
Change-Id: I1416f906dcc8c31a528ab7f09a258873fcfe9d1e
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>