agp/intel-gtt: Only register fake agp driver for gen1

agp/intel-gtt: Only register fake agp driver for gen1

The fake agp driver for the intel graphics gart is only needed for ums
support. And we ditched that a long time ago:

commit 15b28339fb9a0cab29207e0f8c6b6b91eccd75c6
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Jul 23 16:27:25 2014 +0200

    drm/i915: Ditch UMS config option

With this there's no longer the problem that 2 drivers (fake agp
driver and the drm/i915 driver) fight over the same piece, which fixes
apparent dma leaks detected by CONFIG_DMA_API_DEBUG.

Note that the leak isn't real since intel-gtt refcounts and will tear
down eventually. But the debug code assumes that when the i915 driver
unbinds from the pci device everything should be gone. Which isn't the
case if we have intel-agp enabled - userspace might need it. But by
ditching this intel-gtt setup and teardown is completely tied to the
livetime of the "real" driver.

While at it untangle the init ordering a bit - the fake agp wouldn't
be initialized correctly if i915.ko loads first. Which isn't a problem
since when i915 loads in kms mode you won't need the fake agp support
needed by the ums driver ...

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93793
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1453901881-26425-3-git-send-email-daniel.vetter@ffwll.ch