drm/radeon: add pm sysfs files late

commit 51a4726b

 upstream.

They were added relatively early in the driver init process
which meant that in some cases the driver was not finished
initializing before external tools tried to use them which
could result in a crash depending on the timing.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>