gallium: enable dmabuf on BSD as well

The DRM_CONF_SHARE_FD code did not check for Linux, so the commit that
introduced PIPE_CAP_DMABUF broke Wayland-EGL clients on FreeBSD.

Fixes: 8ae50e60 (gallium: replace DRM_CONF_SHARE_FD with PIPE_CAP_DMABUF)
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/gallium/auxiliary/util/u_screen.c

@@ -360,7 +360,7 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
       return 1;
 
    case PIPE_CAP_DMABUF:
-#ifdef PIPE_OS_LINUX
+#if defined(PIPE_OS_LINUX) || defined(PIPE_OS_BSD)
       return 1;
 #else
       return 0;