drm/amd/display: Do not add '-mhard-float' to dcn2{1,0}_resource.o for clang

This patch is for linux-5.15.y and earlier only. It is functionally
equivalent to upstream commit 7db038d9790e ("drm/amd/display: Do not add
'-mhard-float' to dml_ccflags for clang"), which was created after all
files that require '-mhard-float' were moved under the dml folder. In
kernels older than 5.18, which do not contain upstream commits

  22f87d998326 ("drm/amd/display: move FPU operations from dcn21 to dml/dcn20 folder")
  cf689e869cf0 ("drm/amd/display: move FPU-related code from dcn20 to dml folder")

newer versions of clang error with

  clang: error: unsupported option '-mhard-float' for target 'x86_64-linux-gnu'
  make[6]: *** [scripts/Makefile.build:289: drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_resource.o] Error 1
  clang: error: unsupported option '-mhard-float' for target 'x86_64-linux-gnu'
  make[6]: *** [scripts/Makefile.build:289: drivers/gpu/drm/amd/amdgpu/../display/dc/dcn21/dcn21_resource.o] Error 1

Apply a functionally equivalent change to prevent adding '-mhard-float'
with clang for these files.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>