drm/amd/display: fix crash with modesetting driver

They might call commit with ACTION_NOTHING without a mode.
We shouldn't crash but simply skip updating stream scaling
settings since scaling obviously didn't change without a
provided mode.

Signed-off-by: Harry Wentland <harry.wentland@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>