drm/i915: add DP test request handling

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: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>