drm/i915: Initialize downclock mode in panel init

Instead of modifying intel_panel in lvds_init_connector/dsi_init/
edp_init_connector, making changes to move intel_panel->downclock_mode
initialization to intel_panel_init()

v2: Jani's review comments incorporated
Removed downclock_mode local variable in dsi_init and
edp_init_connector

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Pradeep Bhat <pradeep.bhat@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>