drm/amd/display: add DCN support for aarch64

This adds ARM64 support into the DCN. This mainly enables support
for Navi graphics cards. The dcn10 changes haven't been tested,
since I don't have the relevant hardware available, but there
is no way to conditionally disable them, so I've done them anyway.

Signed-off-by: Daniel Kolesa <daniel@octaforge.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>