drm/amd/display: Free memory allocation

[WHY]

Free memory to avoid memory leak

Reviewed-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Clayton King <clayton.king@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>