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>

drivers/gpu/drm/amd/amdgpu/si_dpm.c

@@ -4098,14 +4098,13 @@ static int si_notify_smc_display_change(struct amdgpu_device *adev,
 
 static void si_program_response_times(struct amdgpu_device *adev)
 {
-	u32 voltage_response_time, backbias_response_time, acpi_delay_time, vbi_time_out;
+	u32 voltage_response_time, acpi_delay_time, vbi_time_out;
 	u32 vddc_dly, acpi_dly, vbi_dly;
 	u32 reference_clock;
 
 	si_write_smc_soft_register(adev, SI_SMC_SOFT_REGISTER_mvdd_chg_time, 1);
 
 	voltage_response_time = (u32)adev->pm.dpm.voltage_response_time;
-	backbias_response_time = (u32)adev->pm.dpm.backbias_response_time;
 
 	if (voltage_response_time == 0)
 		voltage_response_time = 1000;