Hide the way device info is stored, in preparation of making device info
a pointer to the const rodata in i915_pci.c. No functional changes.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3cd626f248c0d6638f1288938bbb577a12286050.1546267488.git.jani.nikula@intel.com