drm/i915: Clean up VLV/CHV maxfifo watermark setup

drm/i915: Clean up VLV/CHV maxfifo watermark setup

Let's compute the maxfifo watermarks using max() instead of min().
Can't even recall why I did it the other way originally. Anyways
using max() avoids having to initialize the watermarks to the max
value first.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>