drm/i915/display: Save a few bytes of memory in intel_backlight_device_register()

'name' may still be "intel_backlight" when backlight_device_register()
is called. In such a case, using kstrdup_const() saves a memory
duplication when dev_set_name() is called in
backlight_device_register().

Use kfree_const() accordingly.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ecfdb3af5005e05131e2fb93fd870830f39a8c29.1708708142.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jani Nikula <jani.nikula@intel.com>