drm/i915/dp: increase native aux defer retry timeout

Give more slack to sink devices before retrying on native aux
defer. AFAICT the 100 us timeout was not based on the DP spec.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Cc: stable@vger.kernel.org (on Jani's request)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>