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

Hugues Fruchet requested to merge hugues.fruchet/gstreamer:waylandsink_dumb into main

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 (closed)

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