drm/i915: Lift has-pinned-pages assert to caller of ____i915_gem_object_get_pages

i915_gem_object_attach_phys() is trying to swap out its shmemfs pages
for a new set of physically contiguous pages, but unfortunately triggers
an assert inside get-pages.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102561
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170906135220.13508-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -2479,8 +2479,6 @@ static int ____i915_gem_object_get_pages(struct drm_i915_gem_object *obj)
 {
 	struct sg_table *pages;
 
-	GEM_BUG_ON(i915_gem_object_has_pinned_pages(obj));
-
 	if (unlikely(obj->mm.madv != I915_MADV_WILLNEED)) {
 		DRM_DEBUG("Attempting to obtain a purgeable object\n");
 		return -EFAULT;
@@ -2510,6 +2508,8 @@ int __i915_gem_object_get_pages(struct drm_i915_gem_object *obj)
 		return err;
 
 	if (unlikely(IS_ERR_OR_NULL(obj->mm.pages))) {
+		GEM_BUG_ON(i915_gem_object_has_pinned_pages(obj));
+
 		err = ____i915_gem_object_get_pages(obj);
 		if (err)
 			goto unlock;
@@ -2593,6 +2593,8 @@ void *i915_gem_object_pin_map(struct drm_i915_gem_object *obj,
 
 	if (!atomic_inc_not_zero(&obj->mm.pages_pin_count)) {
 		if (unlikely(IS_ERR_OR_NULL(obj->mm.pages))) {
+			GEM_BUG_ON(i915_gem_object_has_pinned_pages(obj));
+
 			ret = ____i915_gem_object_get_pages(obj);
 			if (ret)
 				goto err_unlock;