drm/i810: drop device_is_agp callback

Use the same trick we used for i915 when we still had ums support:
Just initialize the agp support unconditionally in the driver load
function.

Unfortunately that means we need to export drm_agp_init again, but I
think that's a lesser evil.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-9-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_agpsupport.c

@@ -421,6 +421,8 @@ struct drm_agp_head *drm_agp_init(struct drm_device *dev)
 	head->base = head->agp_info.aper_base;
 	return head;
 }
+/* Only exported for i810.ko */
+EXPORT_SYMBOL(drm_agp_init);
 
 /**
  * drm_legacy_agp_clear - Clear AGP resource list