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