drm/i915: detect wrong MCH watermark values

Some early bios versions seem to ship with the wrong tuning values for
the MCH, possible resulting in pipe underruns under load. Especially
on DP outputs this can lead to black screen, since DP really doesn't
like an occasional whack from an underrun.

Unfortunately the registers seem to be locked after boot, so the only
thing we can do is politely point out issues and suggest a BIOS
upgrade.

Arthur Runyan pointed us at this issue while discussion DP bugs - thus
far no confirmation from a bug report yet that it helps. But at least
some of my machines here have wrong values, so this might be useful in
understanding bug reports.

v2: After a bit more discussion with Art and Ben we've decided to only
the check the watermark values, since the OREF ones could be be a
notch more aggressive on certain machines.

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Runyan, Arthur J <arthur.j.runyan@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>