drm/i915: Properly terminate KMS mode name string during tv init

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454071949-24677-4-git-send-email-imre.deak@intel.com