drm/i915: Bump timeout for wait_for_engines()

Extract the timeout we use in i915_gem_idle_work_handler() and reuse it
for wait_for_engines() in i915_gem_wait_for_idle(). It too has the same
problem in sometimes having to wait for an extended period before the HW
settles, so make use of the same timeout.

References: 5427f207

 ("drm/i915: Bump wait-times for the final CS interrupt before parking")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171211194135.27095-1-chris@chris-wilson.co.uk