fbdev: sh_mobile_hdmi: Use devm_kzalloc()

Use devm_kzalloc() instead of kzalloc() in order to be free
automatically. This makes cleanup paths more simple.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>