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>