drm/i915: SDVO hotplug have different interrupt status bits for i915/i965/g4x

Note that gen3 is the only platform where we've got the bit
definitions right, hence the workaround of disabling sdvo hotplug
support on i945g/gm is not due to misdiagnosis of broken hotplug irq
handling ...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: add some blurb about sdvo hotplug fail on i945g/gm I've
wondered about while reviewing.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>