drm/fsl-dcu: mask all interrupts on initialization

The state of the interrupt mask register on initialization is
unknown, e.g. U-Boot could already used the DCU. So depending on
the boot loader, the outcome of the interrupt mask register could
be different. A defined state is much more preferable. Also, there
is no value in keeping interrupts enabled which we don't need.
Therefor, mask all interrupts on initialization.

Signed-off-by: Stefan Agner <stefan@agner.ch>