drm/amdgpu: Re-enable DCN for 64-bit powerpc

Commit d11219ad ("amdgpu: disable powerpc support for the newer
display engine") disabled the DCN driver for all of powerpc due to
unresolved build failures with some compilers.

Further digging shows that the build failures only occur with compilers
that default to 64-bit long double.

Both the ppc64 and ppc64le ABIs define long double to be 128-bits, but
there are compilers in the wild that default to 64-bits. The compilers
provided by the major distros (Fedora, Ubuntu) default to 128-bits and
are not affected by the build failure.

There is a compiler flag to force 128-bit long double, which may be the
correct long term fix, but as an interim fix only allow building the DCN
driver if long double is 128-bits by default.

The bisection in commit d11219ad must have gone off the rails at
some point, the build failure occurs all the way back to the original
commit that enabled DCN support on powerpc, at least with some
toolchains.

Depends-on: d11219ad ("amdgpu: disable powerpc support for the newer display engine")
Fixes: 16a9dea1 ("amdgpu: Enable initial DCN support on POWER")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Dan Horák <dan@danny.cz>
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2100
Link: https://lore.kernel.org/r/20220725123918.1903255-1-mpe@ellerman.id.au