drm/amd/display: Check if registers are available before accessing

Check if VERT_FILTER_INIT_BOT and BLACK_OFFSET registers
exists in the DCN SCL IP block before trying to access.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>