drm/i915: Stop reporting error details in dmesg as well as the error-state

drm/i915: Stop reporting error details in dmesg as well as the error-state

As we already capture all the information from the registers into the
error-state, also dumping that to dmesg just generates noise that upsets
CI and users alike (and doesn't provide us with any more information).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161019125203.28851-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>