drm/i915/sdvo: Only use the SDVO pin if it is in the valid range

BIOSes. Can't live without them (apparently), definitely can't live with
them.

Reported-by: Ben Gamari <bgamari@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=24312


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>