drm/amdgpu: Wrap -Wunused-but-set-variable in cc-option

-Wunused-but-set-variable was only supported in clang starting with
13.0.0, so earlier versions will emit a warning, which is turned into a
hard error for the kernel to mirror GCC:

  error: unknown warning option '-Wunused-but-set-variable'; did you mean '-Wunused-const-variable'? [-Werror,-Wunknown-warning-option]

The minimum supported version of clang for building the kernel is
11.0.0, so match the rest of the kernel and wrap
-Wunused-but-set-variable in a cc-option call, so that it is only used
when supported by the compiler.

Closes: https://github.com/ClangBuiltLinux/linux/issues/1869


Fixes: 1b320ad3 ("drm/amd/amdgpu: introduce DRM_AMDGPU_WERROR")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>