drm: Use idr_init_base(1) when using id==0 for invalid

Use the new idr_init_base() function to create an IDR that knows id==0
is never allocated as it maps to an invalid identifier. By knowing that
id==0 is invalid, the IDR can start from id=1 instead avoiding the issue
of having to start each lookup from the zeroth leaf as id==0 is always
unused (and thus the tree-of-bitmaps indicate that is the first
available).

References: 6ce711f27500 ("idr: Make 1-based IDRs more efficient")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian Konig <christian.koenig@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Christian König <christian.koenig@amd.com> as well.
Link: https://patchwork.freedesktop.org/patch/msgid/20180212145533.30046-1-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_gem.c

@@ -98,7 +98,7 @@ drm_gem_init(struct drm_device *dev)
 	struct drm_vma_offset_manager *vma_offset_manager;
 
 	mutex_init(&dev->object_name_lock);
-	idr_init(&dev->object_name_idr);
+	idr_init_base(&dev->object_name_idr, 1);
 
 	vma_offset_manager = kzalloc(sizeof(*vma_offset_manager), GFP_KERNEL);
 	if (!vma_offset_manager) {
@@ -776,7 +776,7 @@ drm_gem_open_ioctl(struct drm_device *dev, void *data,
 void
 drm_gem_open(struct drm_device *dev, struct drm_file *file_private)
 {
-	idr_init(&file_private->object_idr);
+	idr_init_base(&file_private->object_idr, 1);
 	spin_lock_init(&file_private->table_lock);
 }
 

drivers/gpu/drm/drm_syncobj.c

@@ -546,7 +546,7 @@ err_put_fd:
 void
 drm_syncobj_open(struct drm_file *file_private)
 {
-	idr_init(&file_private->syncobj_idr);
+	idr_init_base(&file_private->syncobj_idr, 1);
 	spin_lock_init(&file_private->syncobj_table_lock);
 }