drm/i915/crt: Do not rely upon the HPD presence pin

Whilst most monitors do wire up the HPD presence pin, it seems quite a
few KVM do not. Therefore if we simply rely on the HPD pin being
asserted to indicate a connected monitor we fail miserable, so fall back
to performing a DCC query for the EDID.

Reported-and-tested-by: Matthieu LAVIE <boiteamadmax@hotmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50501


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>