DSS2: Check fclk limits when configuring video planes

Check that the currect functional clock is fast enough to support
the requested scaling ratios. Also check if 5-tap filtering can be
used even though the downscaling ratio is less than 1:2 since the
functional clock rate required for 5-tap filtering can be less than
the requirement for 3-tap filtering, and 5-tap filtering should look
better.

Signed-off-by: Ville Syrjälä <ville.syrjala@nokia.com>