drm/i915: don't use dev->agp

This single leftover use is due to a patch that went into 3.5 through
-fixes. With the fake agp stuff on demise, at least for gen6+ we can't
use this any more.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>