drm/amdgpu/display: FP fixes for DCN3.x (v4)

Add proper FP_START/END handling and adjust Makefiles per
previous asics.

v2: fix up harder.
v3: fix clkmgr Makefile for dcn30
v4: fix old gcc handling is only required for x86

Build-tested-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com> (v1)
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>