drm/i915/crt: Explicitly return false if connected to a digital monitor

Rather than proceed on and silently return false by default, mention why
we rejected the presence of an EDID as implying the presence of a VGA
monitor. (The question arises whether there is a broken EDID which falsely
reports a digital connection when attached by VGA.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>