drm/amd/display: Complete TODO item: use new DRM iterator

Abandon new_crtcs array and use for_each_new iterator to acquire new
crtcs.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>