Compositor: Fix refresh rate

Wayland output defines refresh rate in millihertz (int) while Qt uses Hz
(float). Make sure all integer APIs are using millihertz, and covert Hz
values to mHz when passing from Qt.

Change-Id: I5886f5618680d99db6fc106dd7b8998b00a2face
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>