drm/i915: Enable SDVO hotplug interrupts for HDMI and DVI

I was seeing a nasty 5 frame glitch every 10 seconds, caused by the
poll for connection on DVI attached by SDVO.

As my SDVO DVI supports hotplug detect interrupts, the fix is to
enable them, and hook them in to the various bits of driver
infrastructure so that they work reliably.

Note that this is only tested on single-function DVI-D SDVOs, on two
platforms (965GME and 945GSE), and has not been checked against a
specification document.

With lots of help from Adam Jackson <ajax@redhat.com> on IRC.

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>