ACPI: ibm-acpi: backlight device cleanup

This patch cleans up the recently added backlight device support by Holger
Macht <hmacht@suse.de> to fit well with the rest of the code, using the
ibms struct as the other "subdrivers" in ibm-acpi.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>