drm/i915: add const to bin_attribute

Add const to bin_attribute structures as they are only passed to the
functions sysfs_{remove/create}_bin_file or
device_{remove/create}_bin_file. The corresponding arguments are of
type const, so declare the structures to be const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1501694447-14356-1-git-send-email-bhumirks@gmail.com