drm/amdgpu: remove set but not used variable 'vbi_time_out'

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

drivers/gpu/drm/amd/amdgpu/si_dpm.c: In function 'si_program_response_times':
drivers/gpu/drm/amd/amdgpu/si_dpm.c:4101:29: warning:
 variable 'backbias_response_time' set but not used [-Wunused-but-set-variable]

It's never used since introduction in 841686df ("drm/amdgpu: add SI DPM
support (v4)"), so can be removed

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>