drm/radeon: Add support for the ATIF ACPI method to the radeon driver

By calling the ATIF method in the radeon driver we can make sure
that hotkeys such as the video switch key emit ACPI events when
pressed.

agd5f: fix warning

Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>