drm/amd/display: Allow building DC with clang on LoongArch

Clang on LoongArch (18+) appears to be unaffected by the bug causing
excessive stack usage in calculate_bandwidth(). But when building DC_FP
support the stack frame size can be as large as 2816 bytes, which causes
the FRAME_WARN build warnings. So on LoongArch we allow building DC with
clang, but disable DC_FP by default.

The help message is also updated.

Tested-by: Rui Wang <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>