During modesetting, we need to wait for the hardware to report
readiness by polling the registers. Normally, we call msleep() between
reads, because some state changes may take a whole vblank or more
to complete. However during a panic, we are in an atomic context and
cannot sleep. Instead, busy spin polling the termination condition.

References: https://bugzilla.kernel.org/show_bug.cgi?id=31772Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>