drm/amd/display: add DCN support for ARM64

After moving all FPU code to the DML folder, we can enable DCN support
for the ARM64 platform. Remove the -mgeneral-regs-only CFLAG from the
code in the DML folder that needs to use hardware FPU, and add a control
mechanism for ARM Neon.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Ao Zhong <hacc1225@gmail.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>