DPCD 1.1+ adds some automated test infrastructure support.  Add support
for reading the IRQ source and jumping to a test handling routine if
needed.  Subsequent patches will handle particular tests; this patch
just ACKs any requested tests by default.
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NKeith Packard <keithp@keithp.com>