drm/i915: Kill legeacy AGP for gen3 kms

Thus far we've tried to carefully work around the fact that old
userspace relied on the AGP-backed legacy buffer mapping ioctls for a
bit too long. But it's really horribly, and now some new users for it
started to show up again:

http://www.mail-archive.com/mesa-dev@lists.freedesktop.org/msg45547.html

This uses drmAgpSize to figure out the GTT size, which is both the
wrong thing to inquire and also might force us to keep this crap
around for another few years.

So I want to stop this particular zombie from raising ever again. Now
it's only been 4 years since XvMC was fixed for gen3, so a bit early
by the usual rules. But since Linus explicitly said that an ABI
breakage only counts if someone actually observes it I want to tempt
fate an accelarate the demise of AGP.

We probably need to wait 2-3 kernel releases with this shipping until
we go on a killing spree code-wise.

v2: Remove intel_agp_enabled since it's unused (Ville).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>