amdgpu: Enable initial DCN support on POWER

DCN requires floating point support to operate.  Add the appropriate
x86/ppc64 guards and FPU / AltiVec / VSX context switches to DCN.

Note that the current DC20 code doesn't contain all required FPU
wrappers on x86 or POWER, so this patch is insufficient to fully
enable DC20 on POWER.

v2: s/X86_64/X86/g to retain previous behavior.

Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>