drm/i915: Fix channel ending action for DP aux transaction

We should use current channel 'status' bits to clear DP aux channel's
done and error bits, instead of using the channel setting bits, that
will set send/busy bit again to initiate new transaction.

This also includes also some minor cleanup.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>