drm/i915: shorten debugfs output simple attributes

commit 647416f9
Author: Kees Cook <keescook@chromium.org>
Date:   Sun Mar 10 14:10:06 2013 -0700

    drm/i915: use simple attribute in debugfs routines

made i915_next_seqno debugfs entry to crop it's output
if returned value was large enough. Using simple_attr
will limit the output to 24 bytes.

Fix is to strip out preamples on all simple attributes
that have one.

v2: Fix all simple attributes (Daniel Vetter)

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>