drm/sun4i: hdmi: Fix double flag assignation

The is_double flag is a boolean currently assigned to the value of the d
variable, that is either 1 or 2. It means that this is_double variable is
always set to true, even though the initial intent was to have it set to
true when d is 2.

Fix this.

Fixes: 9c568101 ("drm/sun4i: Add HDMI support")
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/20181021163446.29135-2-maxime.ripard@bootlin.com