drm/atmel-hlcdc: revert shift by 8

Revert shift by 8 of state->base.alpha. This introduced a
regression on planes.

Fixes: 7f73c10b

 ("drm/atmel-hclcdc: Convert to the new generic alpha property")
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1556195748-11106-7-git-send-email-claudiu.beznea@microchip.com