drm/amd/display: Move lock to front end program.

Moved lock and unlock to apply_ctx_to_surface, since all
the front end programming is within apply_ctx_to_surface.

Signed-off-by: Yongqiang Sun <yongqiang.sun@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>