UBUNTU: SAUCE: platform/x86: dell-uart-backlight: add get_display_mode command

UBUNTU: SAUCE: platform/x86: dell-uart-backlight: add get_display_mode command

BugLink: https://bugs.launchpad.net/bugs/1865402
ODM asks us to use get_display_mode command to confirm the scalar's
behavior, and Windows use this command, too.
To align the behavior with Windows, remove get_scalar_status command and
replace it with get_display_mode.

Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>