drm/amd/display: Avoid full modeset when not required

Fix IGT test case (kms_atomic_transition)
-DRM sets the mode_changed flag, while we don't need to do a full
 modeset.
-We want to override the mode_changed flag in this case

If we don't do this, we will still bypass the modeset in DC. This will
fail to update the new stream_status, causing nullptr at a later stage
when trying to access stream_status"

We now avoid this by discarding the new_stream instead of partially
filling it.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>