drm/i915: fix 845G FIFO size & burst length

I had one report of flicker due to FIFO underruns on 845G.  Scott was
kind enough to test a few patches and report success with this one.
Looks like 845G measures FIFO size slightly differently than other
chips, and we were also clobbering the FIFO burst length.  Fixing both
of those issues gives him a healthy machine again.

Note that we still only adjust plane A's watermark in the 830/845
case.  If someone is willing to test we could support a bigger variety
of dual-head 830/845 configurations with a bit more code.

Fixes fdo bug #19304 (again).

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