amdgpu: disable powerpc support for the newer display engine

The DRM_AMD_DC_DCN display engine support (Raven, Navi, and newer) has
not been building cleanly on powerpc and causes link errors due to
mixing hard- and soft-float object files:

  powerpc64-linux-ld: drivers/gpu/drm/amd/amdgpu/../display/dc/dml/display_mode_lib.o uses hard float, drivers/gpu/drm/amd/amdgpu/../display/dc/dcn31/dcn31_resource.o uses soft float
  powerpc64-linux-ld: failed to merge target specific data of file drivers/gpu/drm/amd/amdgpu/../display/dc/dcn31/dcn31_resource.o
  [..]

and while patches are floating around, it's not exactly obvious what is
going on.

The problem bisects to commit 41b7a347 ("powerpc: Book3S 64-bit
outline-only KASAN support") but that is probably more about changing
config variables than the fundamental cause.

Despite the bisection result, a more directly related commit seems to be
26f4712a ("drm/amd/display: move FPU related code from dcn31 to
dml/dcn31 folder").  It's probably a combination of the two.

This has been going on since the merge window, without any final word.
So instead of blindly applying patches that may or may not be the right
thing, let's disable this for now.

As Michael Ellerman says:
 "IIUIC this code was never enabled on ppc before, so disabling it seems
  like a reasonable fix to get the build clean"

and once we have more actual feedback (and find any potential users) we
can always re-enable it with the patch that fixes the issues and
back-port as necessary.

Fixes: 41b7a347 ("powerpc: Book3S 64-bit outline-only KASAN support")
Fixes: 26f4712a ("drm/amd/display: move FPU related code from dcn31 to dml/dcn31 folder")
Reported-and-tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/all/20220606153910.GA1773067@roeck-us.net/
Link: https://lore.kernel.org/all/20220618232737.2036722-1-linux@roeck-us.net/
Link: https://lore.kernel.org/all/20220713050724.GA2471738@roeck-us.net/Acked-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/amd/display/Kconfig

@@ -6,7 +6,7 @@ config DRM_AMD_DC
 	bool "AMD DC - Enable new display engine"
 	default y
 	select SND_HDA_COMPONENT if SND_HDA_CORE
-	select DRM_AMD_DC_DCN if (X86 || PPC64) && !(KCOV_INSTRUMENT_ALL && KCOV_ENABLE_COMPARISONS)
+	select DRM_AMD_DC_DCN if X86 && !(KCOV_INSTRUMENT_ALL && KCOV_ENABLE_COMPARISONS)
 	help
 	  Choose this option if you want to use the new display engine
 	  support for AMDGPU. This adds required support for Vega and