llvmpipe: query winsys support for dmabuf mapping

Fixes #11257 by ensuring winsys mapping functions is only called
if its supported by the winsys, which should prevent llvmpipe from
crashing with kmswast.

If the winsys is kms_swrast then this method will be null, but on
drisw it will be available.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29546>

src/gallium/drivers/llvmpipe/lp_texture.c

@@ -664,7 +664,13 @@ llvmpipe_resource_from_handle(struct pipe_screen *_screen,
 #ifdef HAVE_LINUX_UDMABUF_H
       struct llvmpipe_memory_fd_alloc *alloc;
       uint64_t size;
-      if(_screen->import_memory_fd(_screen, whandle->handle, (struct pipe_memory_allocation**)&alloc, &size, true)) {
+      /* Not all winsys implement displaytarget_create_mapped so we need to check
+       * that is available (not null).
+       */
+      if (winsys->displaytarget_create_mapped &&
+          _screen->import_memory_fd(_screen, whandle->handle,
+                                    (struct pipe_memory_allocation**)&alloc,
+                                    &size, true)) {
          data = alloc->data;
          lpr->dt = winsys->displaytarget_create_mapped(winsys, template->bind,
                                                        template->format, template->width0, template->height0,