powerpc/configs: Turn CONFIG_R128 back in pmac32_defconfig

In commit d92d9c3a

 ("drm: hide legacy drivers with
CONFIG_DRM_LEGACY") CONFIG_R128 was moved behind CONFIG_DRM_LEGACY
meaning it stopped being enabled by pmac32_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>