drm/i915/crt: Check for a analog monitor in case of DVI-I

Since Linux 2.6.36 the digital output on my system (855GME + DVI-I) is
not working any longer. The analog output is always activated
regardless of the type of monitor attached.

The culprit seems to be intel_crt_detect_ddc(), which returns true as
soon as an ACK from the EDID device is received. Obviously this
approach does not work with DVI-I where the analog and digital outputs
share a common DDC bus.

In a similar manner to the shared DDC wire, ala the "Mac Mini Hack", we
need an additional check to make sure that there really is an analog
device attached to the DDC.

Signed-off-by: David Müller <d.mueller@elsoft.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org