drm/i915: Avoid using msleep under kdb and wait_for()

wait_for() uses msleep() to yield the cpu whilst spinning waiting for a
register to change. kdb asserts that mode changes are atomic and so
prohibits msleep. The alternative would be to use mdelay or to simply
probe the register more often instead of busy waiting.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>