xf86: fix multi-seat video device support. (v2)

If we are not seat 0 the following apply:

don't probe any bus other than platform
don't probe any drivers other than platform
assume the first platform device we match on the bus is the primary GPU.

This just adds checks in the correct places to ensure this, and
with this X can now start on a secondary seat for an output device.

v2: fix Seat0 macros
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

hw/xfree86/common/xf86Bus.c

@@ -81,6 +81,8 @@ xf86CallDriverProbe(DriverPtr drv, Bool detect_only)
     if (drv->platformProbe != NULL) {
         foundScreen = xf86platformProbeDev(drv);
     }
+    if (ServerIsNotSeat0())
+        return foundScreen;
 #endif
 
 #ifdef XSERVER_LIBPCIACCESS
@@ -214,6 +216,8 @@ xf86BusProbe(void)
 {
 #ifdef XSERVER_PLATFORM_BUS
     xf86platformProbe();
+    if (ServerIsNotSeat0())
+        return;
 #endif
 #ifdef XSERVER_LIBPCIACCESS
     xf86PciProbe();

hw/xfree86/common/xf86platformBus.c

@@ -359,6 +359,9 @@ xf86platformProbeDev(DriverPtr drvp)
                     break;
             }
             else {
+                /* for non-seat0 servers assume first device is the master */
+                if (ServerIsNotSeat0())
+                    break;
                 if (xf86_platform_devices[j].pdev) {
                     if (xf86IsPrimaryPlatform(&xf86_platform_devices[j]))
                         break;