radeonfb: remove warning with CONFIG_PM=n

Remove warning from powerpc ppc64_defconfig builds:

drivers/video/aty/radeon_pm.c:30: warning: 'radeon_reinitialize_M10' declared 'static' but never defined

It's used only under CONFIG_PM, and only with CONFIG_X86 before it is
defined, so the forward declaration can be moved under the ifdef.

Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>