Add a return value to load_cursor_argb() to allow it to report failure

Add a return value to load_cursor_argb() to allow it to report failure

load_cursor_argb() may need to be able to fail and have the server fall back
to a software cursor in at least the following circumstances.
1) The hardware can only support some ARGB cursors and this does not just
depend on cursor size.
2) Virtual hardware may not wish to pass through a cursor to the host at a
particular time but may wish to accept the same cursor at another time.
This patch adds a return value to the API and makes the server do the
software fall-back on failure.

Signed-off-by: Michael Thayer <michael.thayer@oracle.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>