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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c

@@ -210,6 +210,8 @@ static int __set_legacy_tf(struct dc_transfer_func *func,
 	res = mod_color_calculate_regamma_params(func, gamma, true, has_rom,
 						 NULL);
 
+	dc_gamma_release(&gamma);
+
 	return res ? 0 : -ENOMEM;
 }