drm/amd/display: Support clang option for stack alignment

As previously fixed for dml in commit 4769278e ("amdgpu/dc/dml:
Support clang option for stack alignment") and calcs in commit
cc32ad8f ("amdgpu/dc/calcs: Support clang option for stack
alignment"), dcn20 uses an option that is not available with clang:

clang: error: unknown argument: '-mpreferred-stack-boundary=4'
scripts/Makefile.build:281: recipe for target 'drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_resource.o' failed

Use the same trick that we have in the other two files.

Fixes: 7ed4e635 ("drm/amd/display: Add DCN2 HW Sequencer and Resource")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>