drm/amdgpu/display: drop set but unused variable

Fixes this warning:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:9143:27: warning: variable 'abo' set but not used [-Wunused-but-set-variable]

Fixes: 047de3f17a83 ("drm/amdgpu: switch DM to atomic fence helpers v2")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220616203538.649041-1-alexander.deucher@amd.com