drm/amdgpu/dc: Really fix DCN3.1 Makefile for PPC64

Also copy over the part that makes old gcc handling cross-platform.

Fixes: df7a1658f2 ("drm/amdgpu/dc: fix DCN3.1 Makefile for PPC64")
Fixes: 926d6972ef ("drm/amd/display: Add DCN3.1 blocks to the DC Makefile")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn31/Makefile

@@ -28,6 +28,7 @@ endif
 CFLAGS_$(AMDDALPATH)/dc/dcn31/dcn31_resource.o += -mhard-float
 endif
 
+ifdef CONFIG_X86
 ifdef IS_OLD_GCC
 # Stack alignment mismatch, proceed with caution.
 # GCC < 7.1 cannot compile code using `double` and -mpreferred-stack-boundary=3
@@ -36,6 +37,7 @@ CFLAGS_$(AMDDALPATH)/dc/dcn31/dcn31_resource.o += -mpreferred-stack-boundary=4
 else
 CFLAGS_$(AMDDALPATH)/dc/dcn31/dcn31_resource.o += -msse2
 endif
+endif
 
 AMD_DAL_DCN31 = $(addprefix $(AMDDALPATH)/dc/dcn31/,$(DCN31))