drm/amd/display: fix issues with driver unload

Currently, we run into a number of WARN()s when attempting to unload the
amdgpu driver (e.g. using "modprobe -r amdgpu"). These all stem from
calling drm_encoder_cleanup() too early. So, to fix this we can stop
calling drm_encoder_cleanup() from amdgpu_dm_fini() and instead have it
be called from amdgpu_dm_encoder_destroy(). Also, we don't need to free
in amdgpu_dm_encoder_destroy() since mst_encoders[] isn't explicitly
allocated by the slab allocator.

Fixes: f74367e4 ("drm/amdgpu/display: create fake mst encoders ahead of time (v4)")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>