radeonsi: fix a crash when binding a sampler buffer

Buffers don't contain r600_texture.

Broken by 7aedbbacae6d3ec3d06735fff2eb66:
"radeonsi: put image, fmask, and sampler descriptors into one array"

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94091

src/gallium/drivers/radeonsi/si_descriptors.c

@@ -203,7 +203,8 @@ static void si_set_sampler_view(struct si_context *sctx,
 		pipe_sampler_view_reference(&views->views[slot], view);
 		memcpy(views->desc.list + slot * 16, rview->state, 8*4);
 
-		if (rtex && rtex->fmask.size) {
+		if (view->texture && view->texture->target != PIPE_BUFFER &&
+		    rtex->fmask.size) {
 			memcpy(views->desc.list + slot*16 + 8,
 			       rview->fmask_state, 8*4);
 		} else {