drm/amd/display: fix dcn21 Makefile for clang

Just like all the other variants, this one passes invalid
compile-time options with clang after the new code got
merged:

clang: error: unknown argument: '-mpreferred-stack-boundary=4'
scripts/Makefile.build:265: recipe for target 'drivers/gpu/drm/amd/amdgpu/../display/dc/dcn21/dcn21_resource.o' failed

Use the same variant that we have for dcn20 to fix compilation.

Fixes: eced51f9 ("drm/amd/display: Add hubp block for Renoir (v2)")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn21/Makefile

@@ -3,7 +3,17 @@
 
 DCN21 = dcn21_hubp.o dcn21_hubbub.o dcn21_resource.o
 
-CFLAGS_$(AMDDALPATH)/dc/dcn21/dcn21_resource.o := -mhard-float -msse -mpreferred-stack-boundary=4
+ifneq ($(call cc-option, -mpreferred-stack-boundary=4),)
+	cc_stack_align := -mpreferred-stack-boundary=4
+else ifneq ($(call cc-option, -mstack-alignment=16),)
+	cc_stack_align := -mstack-alignment=16
+endif
+
+CFLAGS_$(AMDDALPATH)/dc/dcn21/dcn21_resource.o := -mhard-float -msse $(cc_stack_align)
+
+ifdef CONFIG_CC_IS_CLANG
+CFLAGS_$(AMDDALPATH)/dc/dcn21/dcn21_resource.o += -msse2
+endif
 
 AMD_DAL_DCN21 = $(addprefix $(AMDDALPATH)/dc/dcn21/,$(DCN21))