drm/gma500/cdv: Add and hook up chip op for watermarks

Add a callback hook to the chip ops struct to allow chips to have their
specific fifo watermark update function. Currently only cdv actually
tries to set wms based on crtc configuration but if/when the other chips
needs it we can attach a callback for them as well.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>