Kernels older than 3.6 don't have intel_enable_gtt() exported
and most Linux distributions have CONFIG_AGP_INTEL as built-in
(y) and as such we don't bother to make it available as
modular here. The change to export intel_enable_gtt() was
made through commit 8ecd1a66 but previous to this the driver
never called intel_enable_gtt() so we can safely assume we
can do the same for older kernels.

There should be a way to export an intel_enable_gtt() for
kernels with CONFIG_AGP_INTEL as built-in but that itself
is a project.

commit 8ecd1a6615f0d9de6759aafe229bc1cc4ee99c7b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jun 7 15:56:03 2012 +0200

    drm/i915: call intel_enable_gtt
    
    When drm/i915 is in control of the gtt, we need to call
    the enable function at all the relevant places ourselves.
    
    Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3971,8 +3971,10 @@ i915_gem_init_hw(struct drm_device *dev)
 	drm_i915_private_t *dev_priv = dev->dev_private;
 	int ret;
 
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(3,6,0))
 	if (!intel_enable_gtt())
 		return -EIO;
+#endif
 
 	if (IS_HASWELL(dev) && (I915_READ(0x120010) == 1))
 		I915_WRITE(0x9008, I915_READ(0x9008) | 0xf0000);
