drm/amd/display: Do DC mode-change check after stream creation

drm/amd/display: Do DC mode-change check after stream creation

Do DC level mode change checks (via dc_stream_state) only when creating
a new stream, as this check is uneccessary without a new dc_stream_state
anyways. Doing so better demonstrates the intent of this mode-change
check, in comparison to guarding it with the 'enable' flag.

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