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>