drm/i915: Make the fallback IRQ wait not sleep.

The waits we do here are generally so short that sleeping is a bad
idea unless we have an IRQ to wake us up.  Improves regression test
performance from 18 minutes to 3.5 minutes on gen7, which is now
consistent with the previous generation.

Signed-off-by: Eric Anholt <eric@anholt.net>
Tested-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Keith Packard <keithp@keithp.com>