radeon: Conditionally compile PM code

The power management code is only used on X86 and PowerMac. To prevent
the compiler from warning about unused code, only build when PM and one
of X86 or PowerMac is selected.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>