Remove redundant declaration for radeon_pm_acpi_event_handler(..) to fix
-Wredundant-decls warning

MFC after: 1 week
PR: 209924
Reported by: Mark Millard <markmi@dsl-only.net>
Tested with: devel/amd64-gcc (5.3.0)
Sponsored by: EMC / Isilon Storage Division