drm/amd/display: Move iteration out of dm_update_planes

[Why]
To reduce indentation of dm_update_planes, and to make it operate on
single plane instances.

[How]
Move iteration of plane states into atomic_check.
No functional change is intended.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>