drm/i915: Extract intel_edp_backlight_setup()

Pull the eDP backlight setup into its own function. No
reason to pollute intel_edp_init_connector() with all
the mundane details.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220912111814.17466-7-ville.syrjala@linux.intel.com


Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>