llvmpipe: Resolve regressions from !27805

Resolves regressions caused by !27805 (merged), specifically caused by commit d74ea2c1

To fix #10995 (closed) this MR will only use the display fallback that returns a null handle when the display target is not mapped. We need to do this as the DRI code will query the kms handle (which we can't create as the dmabuf is created through udmabuf and not drm) to get properties like width, height, stride, format, etc. The regression was caused since the old code would always use the fallback, but this is incorrect as if the dt is mapped we can call winsys->displaytarget_get_handle

To fix #11032 (closed) the meson file will now define HAVE_LINUX_UDMABUF_H if the udmabuf header is found. We then use this to conditionally compile the udmabuf code. Which should resolve the issue as the code will only be included now if the header file exists.