drm/i915: dp aux irq support for g4x/vlv

Now we have this everywhere. Next up would be to wire up the DP
hotplug pin to speed up panel power sequencing for eDP panels ...

I've decided to leave the has_aux_irq logic in the code, it should
come handy for hw bringup.

For testing/fail-safety the dp aux code already has a timeout when
waiting for interrupts to signal completion and screams rather loud if
they don't arrive in time. Given that we need a real piece of hw to
talk to anyway this is probably as good as it gets.

v2: Don't check the dp aux channel bits on i965 machines, they have a
different meaning there. Yay for reusing bits at will! Spotted by
Jani.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>