drm/i915: Set correct TV detection voltage level override values

We detect TV connect status by setting DAC voltage level override
values as 0.7 voltage for DAC_A/B/C. The corresponding 2-bits shold be 0x2,
In order correctly to set last bit as 0, at first we must clean it.

It fixed freedesktop.org bug #21204

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_tv.c

@@ -1392,6 +1392,9 @@ intel_tv_detect_type (struct drm_crtc *crtc, struct intel_output *intel_output)
 		tv_ctl &= ~TV_TEST_MODE_MASK;
 		tv_ctl |= TV_TEST_MODE_MONITOR_DETECT;
 		tv_dac &= ~TVDAC_SENSE_MASK;
+		tv_dac &= ~DAC_A_MASK;
+		tv_dac &= ~DAC_B_MASK;
+		tv_dac &= ~DAC_C_MASK;
 		tv_dac |= (TVDAC_STATE_CHG_EN |
 			   TVDAC_A_SENSE_CTL |
 			   TVDAC_B_SENSE_CTL |