drm radeon: Return -EINVAL on wrong pm sysfs access

Throw an error if someone tries to fill this with
wrong data, instead of simply ignoring the input.
Now you get:

echo hello >/sys/../power_method
-bash: echo: write error: Invalid argument

Signed-off-by: Thomas Renninger <trenn@suse.de>
CC: Alexander.Deucher@amd.com
CC: dri-devel@lists.freedesktop.org
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>