drm/i915: Round up the watermark entries (v3)

Even though "we have enough padding that it should be ok", round up the
watermark entries to the next unit to be on the safe side...

v2: Use the DIV_ROUND_UP macro
v3: Spotted a few more missing round-ups.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>