drm/i915/dp: read more receiver capability bits on hotplug

When a hotplug event is received, we need to check the receiver cap bits
in case they've changed (as they might with a hub or chain config).

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