drm/amd/display: Do not program front-end twice

The sequence of front-end > back-end > front-end programming will
program the front-end more than once. Add a mode_changed flag, and use
it to determine whether the front-end should be programmed before, or
after back-end.

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