gbm: Give getenv backend override its own function

Signed-off-by: James Jones <jajones@nvidia.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9902>

src/gbm/main/backend.c

@@ -62,8 +62,8 @@ find_backend(const char *name, int fd)
    return dev;
 }
 
-struct gbm_device *
-_gbm_create_device(int fd)
+static struct gbm_device *
+override_backend(int fd)
 {
    struct gbm_device *dev = NULL;
    const char *b;
@@ -72,6 +72,16 @@ _gbm_create_device(int fd)
    if (b)
       dev = find_backend(b, fd);
 
+   return dev;
+}
+
+struct gbm_device *
+_gbm_create_device(int fd)
+{
+   struct gbm_device *dev;
+
+   dev = override_backend(fd);
+
    if (!dev)
       dev = find_backend(NULL, fd);