xwayland/eglstream: allow commits to dma-buf backed pixmaps

As of commit 098e0f52 xwl_glamor_eglstream_allow_commits will not allow commits
if the xwl_pixmap does not have an EGLSurface. This is valid for pixmaps backed
by an EGLStream, however pixmaps backed by a dma-buf for OpenGL or Vulkan
rendering will never have an EGLSurface.  Unlike EGLStream backed pixmaps,
though, glamor will render directly to the buffer that Xwayland passes to the
compositor. Hence, they don't require the intermediate copy in
xwl_glamor_eglstream_post_damage that EGLStream backed pixmaps do, so there is
no need for an EGLSurface.

Signed-off-by: Erik Kurzinger <ekurzinger@nvidia.com>
Acked-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>