drm/i915: Introduce i915_ggtt_offset()

This little helper only exists to safely discard the upper unused 32bits
of the general 64-bit VMA address - as we know that all Global GTT
currently are less than 4GiB in size and so that the upper bits must be
zero. In many places, we use a u32 for the global GTT offset and we want
to document where we are discarding the full VMA offset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-28-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 73f3466..019df75 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -2008,7 +2008,7 @@
 
 	if (vma->flags & I915_VMA_GLOBAL_BIND)
 		seq_printf(m, "\tBound in GGTT at 0x%08x\n",
-			   lower_32_bits(vma->node.start));
+			   i915_ggtt_offset(vma));
 
 	if (i915_gem_object_get_pages(vma->obj)) {
 		seq_puts(m, "\tFailed to get pages for context object\n\n");