drm/sun4i: Set the coef_rdy bit right after the coef have been set

The COEF_RDY bit is used to tell the hardware that new FIR filters
coefficients have been written to the registers and that the hardware
should take them into account starting next frame.

Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190118145133.21281-20-paul.kocialkowski@bootlin.com