drm/i915: Make AGP support optional

We only depend on the intel-gtt module for GTT frobbign on older gens.
The intel_agp module is optional, except for UMS and some old XvMC
userland on gen3. So make AGP support optional. As before, we will
fail the i915 init for UMS and gen3 KMS the same as before if
intel_agp isn't around.

intel-gtt.c is left with a somewhat ugly ifdef mess, but I'm going
to save that for a later cleaning.

At least my gen2 still works with the patch and CONFIG_AGP=n.

v2: Make i915 depend on X86 and PCI, and intel-gtt depend on PCI

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>