drm/radeon: fix possible segfault when parsing pm tables

commit f8e6bfc2 upstream.

If we have a empty power table, bail early and allocate
the default power state.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=63865



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