drm/amd/display: Remove dead enable_plane function definition and call

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Jordan Lazare <Jordan.Lazare@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>