drm/i915: Replace the static panel_type variable with dev_priv->vbt.panel_type

Store the extracted panel_type under dev_priv.vbt instead of keeping
around a static variable for it.

Cc: Rob Kramer <rob@solution-space.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>