drm/i915: use simple attribute in debugfs routines

This replaces the manual read/write routines in debugfs with the common
simple attribute helpers. Doing this gets rid of repeated copy/pasting
of copy_from_user and value formatting code.
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: Squash in follow-up fix from Kees Cook to fix u64 divides on
32bit platforms.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>