drm/amd/display: Don't access legacy properties

We're an atomic driver and shouldn't access legacy properties. Doing so
will only scare users with stack traces.

Instead save the prop in the state and access it directly. Much simpler.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>