drm/amd/display: Clear stream->mode_changed after commit

[Why]
The stream->mode_changed flag can persist in the following sequence
of atomic commits:

Commit 1:
Enable CRTC0 (mode_changed = true), Enable CRTC1 (mode_changed = true)

Commit 2:
Disable CRTC1 (mode_changed = false)

In this sequence we want to keep the exiting CRTC0 but it's not in the
atomic state for the commit since it hasn't been modified. In this case
the stream->mode_changed flag persists as true and we don't re-program
the planes for the existing stream.

[How]
The flag needs to be cleared and it makes the most sense to do it within
DC after the state has been committed. Nothing following dc_commit_state
should think that the stream's mode has changed.

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