drm/i915: Don't leak VBT mode data

We allocate memory for LVDS modes while parsing the VBT at startup, but
never free this memory when the driver is unloaded, causing a small
leak.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>