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 27da3bdf
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
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 <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>