egl/drm: ensure the backing gbm is set before using it

Currently, if we error out before gbm_dri is set (say due to a different
name of the backing GBM implementation, or otherwise) the tear down will
trigger a NULL ptr deref and crash out.

Move the gbm_dri initialization as early as possible.

v2: Drop check in dri2_teardowm_drm (Eric)

Reported-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/egl/drivers/dri2/platform_drm.c

@@ -715,6 +715,7 @@ dri2_initialize_drm(_EGLDriver *drv, _EGLDisplay *disp)
          goto cleanup;
       }
    }
+   dri2_dpy->gbm_dri = gbm_dri_device(gbm);
 
    if (strcmp(gbm_device_get_backend_name(gbm), "drm") != 0) {
       err = "DRI2: gbm device using incorrect/incompatible backend";
@@ -729,7 +730,6 @@ dri2_initialize_drm(_EGLDriver *drv, _EGLDisplay *disp)
 
    disp->Device = dev;
 
-   dri2_dpy->gbm_dri = gbm_dri_device(gbm);
    dri2_dpy->driver_name = strdup(dri2_dpy->gbm_dri->driver_name);
 
    dri2_dpy->dri_screen = dri2_dpy->gbm_dri->screen;