drm/amd/display: Move iteration out of dm_update_crtcs

[Why]
To reduce indent in dm_update_crtcs, and to make it operate on single
instances.

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

Signed-off-by: Leo Li <[email protected]>
Reviewed-by: Nicholas Kazlauskas <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>