drm: provide agp dummies for CONFIG_AGP=n

We currently rely on gcc dead-code elimination so the drm_agp_* helpers
are not called if drm_core_has_AGP() is false. That's ugly as hell so
provide "static inline" dummies for the case that AGP is disabled.

Fixes a build-regression introduced by:

  commit 28ec711c
  Author: David Herrmann <dh.herrmann@gmail.com>
  Date:   Sat Jul 27 16:37:00 2013 +0200

      drm/agp: move AGP cleanup paths to drm_agpsupport.c

v2: switch #ifdef -> #if (spotted by Stephen)

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>