vga_switcheroo: Use VGA_SWITCHEROO_UNKNOWN_ID instead of -1

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/linux/vga_switcheroo.h b/include/linux/vga_switcheroo.h
index e636617..88909a8 100644
--- a/include/linux/vga_switcheroo.h
+++ b/include/linux/vga_switcheroo.h
@@ -59,6 +59,9 @@
 
 /**
  * enum vga_switcheroo_client_id - client identifier
+ * @VGA_SWITCHEROO_UNKNOWN_ID: initial identifier assigned to vga clients.
+ * 	Determining the id requires the handler, so GPUs are given their
+ * 	true id in a delayed fashion in vga_switcheroo_enable()
  * @VGA_SWITCHEROO_IGD: integrated graphics device
  * @VGA_SWITCHEROO_DIS: discrete graphics device
  * @VGA_SWITCHEROO_MAX_CLIENTS: currently no more than two GPUs are supported
@@ -66,6 +69,7 @@
  * Client identifier. Audio clients use the same identifier & 0x100.
  */
 enum vga_switcheroo_client_id {
+	VGA_SWITCHEROO_UNKNOWN_ID = -1,
 	VGA_SWITCHEROO_IGD,
 	VGA_SWITCHEROO_DIS,
 	VGA_SWITCHEROO_MAX_CLIENTS,