[VIDEO] ffb: Fix two DAC handling bugs.

The determination of whether the DAC has inverted cursor logic is
broken, import the version checks the X.org driver uses to fix this.

Next, when we change the timing generator, borrow code from X.org that
does 10 NOP reads of the timing generator register afterwards to make
sure the video-enable transition occurs cleanly.

Finally, use macros for the DAC registers and fields in order to
provide documentation for the next person who reads this code.

Signed-off-by: David S. Miller <davem@davemloft.net>