drm/i915: add timeout to FBC disable waits

FBC disable on 965 can take long enough to trigger latency checks in the
kernel so be sure to timeout after a reasonable period.

Fixes https://bugzilla.kernel.org/show_bug.cgi?id=15015

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Tested-by: James Ettle <theholyettlz@googlemail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>