drm/amd/display: Do DC mode-change check when adding CRTCs

Within atomic check, dm_update_crtcs_state is called twice. First to
remove from the dc_state, and subsequently to add to it.

In both calls, a secondary mode-change check is done using dc-level
states. We shouldn't be doing this while removing, since a new
dc_stream_state has not been created to do the necessary comparison.
Because of this, the mode_changed flag within the DRM state can be
mistakenly set to false. Doing so only when adding prevents this.

We are also guaranteed that a call to add will come after remove, or
else the atomic check fails (and a commit will not happen).

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