drm/i915: honour forced connector modes

In the move over to use BIOS connector configs, we lost the ability to
force a specific set of connectors on or off.  Try to remedy that by
dropping back to the old behavior if we detect a hard coded connector
config that tries to enable a connector (disabling is easy!).

Based on earlier patches by Jesse Barnes.

v2: Remove Jesse's patch

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>