waylandsink: do not use drm dumb pool when importing DMAbuf buffers

There is no need to use DRM dumb pool if buffer to
render is already a DMABuf, just import it and render it.

This fixes a DMAbuf memory leakage when waylandsink downstream
element exports DMABuf while waylandsink is configured to be
DMABuf exporter (drm-device=/drv/dri/card0):

gst-launch-1.0 v4l2src io-mode=4 ! waylandsink drm-device=/dev/dri/card0

leakage identfied with command:
watch "cat /sys/kernel/debug/dma_buf/bufinfo | grep attached "

Fixes #2729

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5350>

subprojects/gst-plugins-bad/ext/wayland/gstwaylandsink.c

@@ -924,12 +924,12 @@ gst_wayland_sink_show_frame (GstVideoSink * vsink, GstBuffer * buffer)
       wbuf = gst_wl_linux_dmabuf_construct_wl_buffer (buffer, self->display,
           &self->drm_info);
 
+    if (!wbuf && !self->skip_dumb_buffer_copy) {
       /* DMABuf did not work, let try and make this a dmabuf, it does not matter
        * if it was a SHM since the compositor needs to copy that anyway, and
        * offloading the compositor from a copy helps maintaining a smoother
        * desktop.
        */
-    if (!self->skip_dumb_buffer_copy) {
       GstVideoFrame src, dst;
 
       if (!gst_wayland_activate_drm_dumb_pool (self)) {