agp: Do not attach to Intel GEN6+

[dumbbell]

Summary

The i915 video driver doesn't depend on agp(4) anymore for Sandybridge and later GPUs. Therefore, there is no need to attach agp(4) to those devices.

kib@, I noticed that the agp_g4x_res_spec variable was used by all GEN6+, but not by G4X. Is is expected?

[kib]

About agp_g4x_res_spec, I think that it actually does not matter. I re-read GMCH datasheets for 965, G45 and SandyBridge. On all three chipsets, device 0:2:0 BAR at 0x10 is combined mmio and GTT, and BAR at 0x18 is the aperture. So the correct thing to do is to add a name for 965 BAR at 0x18 and add a second resource line to agp_i965_res_spec, which should be used by Gen4+.

I think that i965 and g4x res_specs where split due to my mistake.

[kwm]

This patch works fine on my sandybridge system.

[dumbbell]

Fix agp_i965_res_spec

[kib]

Can we leave the AGP_SNB registers definitions in the header ? I find it easier to consult our reg.h than to search datasheets.