drm/i915: Use a temporary va_list for two-pass string handling

In

commit edc3d884
Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Thu May 23 13:55:35 2013 +0300

    drm/i915: avoid big kmallocs on reading error state

we introduce a two-pass mechanism for splitting long strings being
formatted into the error-state. The first pass finds the length, and the
second pass emits the right portion of the string into the accumulation
buffer. Unfortunately we use the same va_list for both passes, resulting
in the second pass reading garbage off the end of the argument list. As
the two passes are only used for boundaries between read() calls, the
corruption is only rarely seen.

This fixes the root cause behind

commit baf27f9b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sat Jun 29 23:26:50 2013 +0100

    drm/i915: Break up the large vsnprintf() in print_error_buffers()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>