drm/i915: mark all device info struct with __initconst

As recommended by Chris.

v2: Switch from __initdata to __initconst.
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170830161208.29221-3-lionel.g.landwerlin@intel.com