drm/sun4i: Protect the TCON pixel clocks

Both TCON clocks are very sensitive to clock changes, since any change
might lead to improper timings.

Make sure our rate is never changed.

Tested-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d5224d2e81ecf73dc09f234e580ada52c00eaee3.1519204731.git-series.maxime.ripard@bootlin.com