drm/i915/dp: Check for AUXCH error before checking for success

This is paranoid, but I am entirely willing to believe the hardware
could come up with a condition where I get a status with both the 'done'
and 'receive error' bits set.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>