drm/i915/dp: try to read receiver capabilities 3 times when detecting

If ->detect is called too soon after a hot plug event, the sink may not
be ready yet.  So try up to 3 times with 1ms sleeps in between tries to
get the data (spec dictates that receivers must be ready to respond within
1ms and that sources should try 3 times).

See section 9.1 of the 1.1a DisplayPort spec.

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