backend-drm: Remove Pixman conditional for keep_buffer

The Pixman renderer keeps its own reference to buffers when attached to
surfaces, through its surface state: just use that instead.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/backend-drm/state-propose.c

@@ -964,9 +964,7 @@ drm_assign_planes(struct weston_output *output_base)
 		if (weston_view_has_valid_buffer(ev)) {
 			struct weston_buffer *buffer =
 				ev->surface->buffer_ref.buffer;
-			if (b->use_pixman)
-				ev->surface->keep_buffer = true;
-			else if (buffer->type == WESTON_BUFFER_DMABUF ||
+			if (buffer->type == WESTON_BUFFER_DMABUF ||
 			    buffer->type == WESTON_BUFFER_RENDERER_OPAQUE)
 				ev->surface->keep_buffer = true;
 			else if (buffer->type == WESTON_BUFFER_SHM &&