powerpc/configs/6xx: Turn CONFIG_DRM_RADEON back on

In commit d92d9c3a ("drm: hide legacy drivers with CONFIG_DRM_LEGACY")
CONFIG_DRM_RADEON was moved behind CONFIG_DRM_LEGACY meaning it
stopped being enabled by ppc6xx_defconfig. Although no one has
noticed, given this is basically a legacy platform, it seems anyone
who is using it probably still wants this driver. So turn it back on
for now.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>