drm/sun4i: tcon-top: Cleanup clock handling

There is no need to acquire reference to clock just to get its name.

This commit just cleans up the code. There is no functional change.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
[Maxime: Fixed the of_property_match_string error check]
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180710203511.18454-5-jernej.skrabec@siol.net