waylandsink: Initialize a KMS BO pointer of the GstWlMeta

This patch fixes the segmentation fault caused by accessing
an uninitialized pointer in gst_wl_meta_free().
waylandsink: Fix compile errors when HAVE_WAYLAND_KMS is not defined
waylandsink: Fix static caps when HAVE_WAYLAND_KMS is disabled

The current implementation supports only xRGB and ARGB when using wl_shm.
waylandsink: Don't set videosink_buffer_creation_request_supported flag when wl_kms is disabled

The videosink_buffer_creation_request query is supported only when
wl_kms is enabled, so the flag shouldn't be set to to the buffer pool
when using wl_shm.