drm/amd/display: fix mode scaling (RMX_.*)

As made mention of in commit 4a2df0d1 ("drm/amd/display: Fixed
non-native modes not lighting up"), we shouldn't call
drm_mode_set_crtcinfo() once the crtc timings have been decided. Since,
it can cause settings to be unintentionally overwritten. So, since
dm_state is never NULL now, we can use old_stream to determine if we
should call drm_mode_set_crtcinfo() because we only need to set the crtc
timing parameters for entirely new streams.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Fixes: bd49f190 ("drm/amd/display: Always set crtcinfo from create_stream_for_sink")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -6026,7 +6026,7 @@ create_stream_for_sink(struct amdgpu_dm_connector *aconnector,
 
 	if (recalculate_timing)
 		drm_mode_set_crtcinfo(&saved_mode, 0);
-	else
+	else if (!old_stream)
 		drm_mode_set_crtcinfo(&mode, 0);
 
 	/*