backlight: lp855x: Add support ACPI enumeration

The Xiaomi Mi Pad 2 tablet uses an ACPI enumerated LP8556 backlight
controller for its LCD-panel, with a Xiaomi specific ACPI HID of
"XMCC0001", add support for this.

Note the new "if (id)" check also fixes a NULL pointer deref when a user
tries to manually bind the driver from sysfs.

When CONFIG_ACPI is disabled acpi_match_device() will always return NULL,
so the lp855x_parse_acpi() call will get optimized away.

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211102225504.18920-3-hdegoede@redhat.com