drm/i915/gtt: Fix unwind length passed to gen6_ppgtt_clear_range

When we want to unwind an error when allocating the PD for gen6, we call
gen6_ppgtt_clear_range() telling to clear upto the PD we've previously
allocated. However, we passed it an incorrect length, passing it the
endpoint instead. Fortunately, as the start was always 0, this has no
impact today, but tomorrow we want to start using non-zero origins.
Reported-by: NMatthew Auld <matthew.william.auld@gmail.com>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: NMatthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180608173221.10455-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1931,7 +1931,7 @@ static int gen6_alloc_va_range(struct i915_address_space *vm,
 	return 0;
 
 unwind_out:
-	gen6_ppgtt_clear_range(vm, from, start);
+	gen6_ppgtt_clear_range(vm, from, start - from);
 	return -ENOMEM;
 }