drm/sun4i: hdmi: Fix unitialized variable

The is_double variable is used to store, and possibly returning to the
calling function, whether it needs to double the rate of the parent clock
or not.

In the case where it does, the variable is affected, but in the case where
it doesn't we return some uninitialized value. Fix this.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181021163446.29135-1-maxime.ripard@bootlin.com