drm/sun4i: Release exclusive clock lock when disabling TCON

Currently exclusive TCON clock lock is never released, which, for
example, prevents changing resolution on HDMI.

In order to fix that, release clock when disabling TCON. TCON is always
disabled first before new mode is set.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180301213442.16677-7-jernej.skrabec@siol.net