drm/amdgpu: remove set but not used variable 'direct_poll' in vcn_v2_0.c

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: Christian König <christian.koenig@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>