drm/i915: disable drm agp support for !gen3 with kms enabled

This is the quick&dirty way Dave Airlie suggested to workaround the
midlayer drm agp brain-damange. Note that i915_probe is only called
when the driver has ksm enabled, so no need to check for that.

We also need to move the intel_agp_enabled check at the right place.
Note that the only thing this does is enforce the correct module load
order (by using a symbol from intel-agp.ko) to ensure that the fake
agp driver is ready before the drm core tries to set up the agp stuff.

v2: Add a comment to explain why gen3 needs all this legacy fake agp
stuff - we've shipped an XvMC library with a kms-enabled ddx that
requires it (but only on gen3).

v3: Make it clear that this is only a gen3 issue in the comment.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>