drm/amdgpu/display: make FP handling in Makefiles consistent

Use the same pattern as the DML Makefile and while we are here
add a missing x86 guard around the msse flags for DCN3.2.x.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>