drm/nouveau: fix init with agpgart-uninorth

Check that the AGP aperture can be mapped. This follows a similar change
done for Radeon (commit 365048ff, drm/radeon: AGP memory is only I/O if
the aperture can be mapped by the CPU.).

The patch fixes the following error seen on G5 iMac:

	nouveau E[     DRM] failed to create kernel channel, -12

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58806
Cc: stable@vger.kernel.org
Reviewed-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -1276,7 +1276,7 @@ nouveau_ttm_io_mem_reserve(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem)
 		if (drm->agp.stat == ENABLED) {
 			mem->bus.offset = mem->start << PAGE_SHIFT;
 			mem->bus.base = drm->agp.base;
-			mem->bus.is_iomem = true;
+			mem->bus.is_iomem = !dev->agp->cant_use_aperture;
 		}
 #endif
 		break;