drm: Document device unplug infrastructure

While at it, also ocd and give them a consistent drm_dev_ prefix, like
the other device instance functionality. Plus move the functions into
the right places.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170802115604.12734-3-daniel.vetter@ffwll.ch
diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index a8d396b..ad4e9cf 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -1001,7 +1001,7 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 	struct drm_vma_offset_node *node;
 	int ret;
 
-	if (drm_device_is_unplugged(dev))
+	if (drm_dev_is_unplugged(dev))
 		return -ENODEV;
 
 	drm_vma_offset_lock_lookup(dev->vma_offset_manager);