drm/i915: Fake AGP is dead

Remove the leftovers, yay!

AGP for i915 kms died long ago with

commit 3bb6ce66
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Nov 13 22:14:16 2013 +0100

    drm/i915: Kill legeacy AGP for gen3 kms

and with ums now gone to there's really no users any more.

Note that device_is_agp is only called when DRIVER_USE_AGP is set and
since we've unconditionally cleared that since a while there are
really no users left for i915_driver_device_is_agp.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>