drm/i915: allow stolen, pre-allocated objects to avoid GTT allocation v2

In some cases, we may not need GTT address space allocated to a stolen
object, so allow passing -1 to the preallocated function to indicate as
much.

v2: remove BUG_ON(gtt_offset & 4095) now that -1 is allowed (Ville)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index 67294f4..6df886e 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -2881,7 +2881,7 @@
 		pcbr_offset = (pcbr & (~4095)) - dev_priv->mm.stolen_base;
 		pctx = i915_gem_object_create_stolen_for_preallocated(dev_priv->dev,
 								      pcbr_offset,
-								      pcbr_offset,
+								      -1,
 								      pctx_size);
 		goto out;
 	}