drm/amd/display: Drop FPU flags from dcn32 Makefile

This is the final commit from the FPU isolation for DCN32 and for this
reason we can finally remove flags related to FPU.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>