drm/i915: only apply crt_present check on VLV

drm/i915: only apply crt_present check on VLV

Apparently we can't trust this field on other platforms and need to find
some other way.

This fixes a regression introduced in
commit 27da3bdfcf7f5233cdfe4563f53edf1ecab7cea0
Author: Jesse Barnes <[email protected]>
Date:   Fri Apr 4 16:12:07 2014 -0700

    drm/i915: use VBT to determine whether to enumerate the VGA port

Signed-off-by: Jesse Barnes <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Tested-by: Alan Stern <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>