Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/vcn_v2_0.c:1848:39: warning: variable ‘direct_poll’ set but not used [-Wunused-but-set-variable]

It is introduced by commit dd26858a ("drm/amdgpu:
implement initialization part on VCN2.0 for SRIOV"), but never used,
so remove it.
Reviewed-by: NChristian König <christian.koenig@amd.com>
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NZheng Bin <zhengbin13@huawei.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>