drm/radeon: remove set but not used variable 'backbias_response_time'

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

drivers/gpu/drm/radeon/si_dpm.c: In function si_program_response_times:
drivers/gpu/drm/radeon/si_dpm.c:3640:29: warning: variable backbias_response_time set but not used [-Wunused-but-set-variable]

It is introduced by commit a9e61410 ("drm/radeon/kms:
add dpm support for SI (v7)"), but never used, so remove it.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/si_dpm.c

@@ -3640,14 +3640,13 @@ static int si_notify_smc_display_change(struct radeon_device *rdev,
 
 static void si_program_response_times(struct radeon_device *rdev)
 {
-	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(rdev, SI_SMC_SOFT_REGISTER_mvdd_chg_time, 1);
 
 	voltage_response_time = (u32)rdev->pm.dpm.voltage_response_time;
-	backbias_response_time = (u32)rdev->pm.dpm.backbias_response_time;
 
 	if (voltage_response_time == 0)
 		voltage_response_time = 1000;