drm/i915: Default to a more lenient forced preemption timeout

Based on a sampling of a number of benchmarks across platforms, by
default opt for a much more lenient timeout so that we should not
adversely affect existing "good" clients.

640ms ought to be enough for anyone.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112169
Fixes: 3a7a92ab

 ("drm/i915/execlists: Force preemption")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191125162737.2161069-1-chris@chris-wilson.co.uk