drm/amd/display: fix non-x86/PPC64 compilation

Need to protect FP DMCUB code with CONFIG_DRM_AMD_DC_DCN.
Fixes build failures like the following on arm64:
ERROR: modpost: "__floatunsidf" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost: "__divdf3" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost: "fma" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost: "__adddf3" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost: "__fixdfsi" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost: "__muldf3" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost: "__floatsidf" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: modpost: "__fixunsdfsi" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!

Fixes: 85f4bc0c ("drm/amd/display: Add SubVP required code")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>