drm/amd/display: fix double free issue during amdgpu module unload

drm/amd/display: fix double free issue during amdgpu module unload

Flexible endpoints use DIGs from available inflexible endpoints,
so only the encoders of inflexible links need to be freed.
Otherwise, a double free issue may occur when unloading the
amdgpu module.

[  279.190523] RIP: 0010:__slab_free+0x152/0x2f0
[  279.190577] Call Trace:
[  279.190580]  <TASK>
[  279.190582]  ? show_regs+0x69/0x80
[  279.190590]  ? die+0x3b/0x90
[  279.190595]  ? do_trap+0xc8/0xe0
[  279.190601]  ? do_error_trap+0x73/0xa0
[  279.190605]  ? __slab_free+0x152/0x2f0
[  279.190609]  ? exc_invalid_op+0x56/0x70
[  279.190616]  ? __slab_free+0x152/0x2f0
[  279.190642]  ? asm_exc_invalid_op+0x1f/0x30
[  279.190648]  ? dcn10_link_encoder_destroy+0x19/0x30 [amdgpu]
[  279.191096]  ? __slab_free+0x152/0x2f0
[  279.191102]  ? dcn10_link_encoder_destroy+0x19/0x30 [amdgpu]
[  279.191469]  kfree+0x260/0x2b0
[  279.191474]  dcn10_link_encoder_destroy+0x19/0x30 [amdgpu]
[  279.191821]  link_destroy+0xd7/0x130 [amdgpu]
[  279.192248]  dc_destruct+0x90/0x270 [amdgpu]
[  279.192666]  dc_destroy+0x19/0x40 [amdgpu]
[  279.193020]  amdgpu_dm_fini+0x16e/0x200 [amdgpu]
[  279.193432]  dm_hw_fini+0x26/0x40 [amdgpu]
[  279.193795]  amdgpu_device_fini_hw+0x24c/0x400 [amdgpu]
[  279.194108]  amdgpu_driver_unload_kms+0x4f/0x70 [amdgpu]
[  279.194436]  amdgpu_pci_remove+0x40/0x80 [amdgpu]
[  279.194632]  pci_device_remove+0x3a/0xa0
[  279.194638]  device_remove+0x40/0x70
[  279.194642]  device_release_driver_internal+0x1ad/0x210
[  279.194647]  driver_detach+0x4e/0xa0
[  279.194650]  bus_remove_driver+0x6f/0xf0
[  279.194653]  driver_unregister+0x33/0x60
[  279.194657]  pci_unregister_driver+0x44/0x90
[  279.194662]  amdgpu_exit+0x19/0x1f0 [amdgpu]
[  279.194939]  __do_sys_delete_module.isra.0+0x198/0x2f0
[  279.194946]  __x64_sys_delete_module+0x16/0x20
[  279.194950]  do_syscall_64+0x58/0x120
[  279.194954]  entry_SYSCALL_64_after_hwframe+0x6e/0x76
[  279.194980]  </TASK>

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Tim Huang <tim.huang@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>