drm: zte: do not enable clock auto-gating by default

Some VOU modules do not work well with clock auto-gating.  For example,
VGA I2C bus will fail to read EDID data from monitor.  Let's not enable
this feature by default, and leave it to the possible future low-power
optimization.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1491490870-6330-2-git-send-email-shawnguo@kernel.org