drm/i915: make sink_crc return -EIO on aux read/write failure

Even though it's unliky, we should check each aux transaction not just
the first one. Also

commit ce31d9f4
Author: Rodrigo Vivi <rodrigo.vivi@intel.com>
Date:   Mon Sep 29 18:29:52 2014 -0400

    drm/i915: preserve other DP_TEST_SINK bits.

added a new aux transaction before the one which was checked. Fix
this.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Todd Previte <tprevite@gmail.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>