drm/i915: handle FIFO oversubsription correctly

If you're pushing a plane hard (i.e. you need most or all of the FIFO
entries just to cover your frame refresh latency), the watermark level
may end up being negative.  So fix up the signed vs. unsigned math in
the calculation function to handle this correctly, giving all available
FIFO entries to such a configuration.

Reported-by: Eric Anholt <eric@anholt.net>
Tested-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>