drm/i915/dp: wait for previous AUX channel activity to clear

Before initiating a new read or write on the DP AUX channel, wait for
any outstanding activity to complete.  This may happen during normal
retry behavior.  If the wait fails (i.e. after 1ms the AUX channel is
still busy) dump a backtrace to make the caller easier to spot.

v2: use msleep instead, and timeout after 3ms (only ever saw 1 retry
    with msleep in testing)
v3: fix backtrace check to trigger if the 3ms wait times out

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=38136

.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>