drm/i915: Pair va_copy with va_end in i915_error_vprintf

Each invocation of va_copy() must be matched by a corresponding
invocation of va_end() in the same function.

This regression has been introduced in

commit e29bb4eb
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Sep 20 10:20:59 2013 +0100

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

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