drm/amd/display: Free gamma after calculating legacy transfer function

[Why]
We're leaking memory by not freeing the gamma used to calculate the
transfer function for legacy gamma.

[How]
Release the gamma after we're done with it.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>