drm/amd/display: Do not add '-mhard-float' to dml_ccflags for clang

When clang's -Qunused-arguments is dropped from KBUILD_CPPFLAGS, it
warns:

clang-16: error: argument unused during compilation: '-mhard-float' [-Werror,-Wunused-command-line-argument]

Similar to commit 84edc2eff827 ("selftest/fpu: avoid clang warning"),
just add this flag to GCC builds. Commit 0f0727d971f6 ("drm/amd/display:
readd -msse2 to prevent Clang from emitting libcalls to undefined SW FP
routines") added '-msse2' to prevent clang from emitting software
floating point routines.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>