drm/i915: reinstate GM45 TV detection fix

This reverts commmit d4b74bf078 which
reverted the origin fix fb8b5a39b6.

We have at least 3 different bug reports that this fixes things and no
indication what is exactly wrong with this. So try again.

To make matters slightly more fun, the commit itself was cc: stable
whereas the revert has not been.

According to Peter Clifton he discussed this with Zhao Yakui and this
seems to be in contradiction of the GM45 PRM, but rumours have it that
this is how the BIOS does it ... let's see.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Tested-by: Peter Clifton <Peter.Clifton@clifton-electronics.com>
Cc: Zhao Yakui <yakui.zhao@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Eric Anholt <eric@anholt.net>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=16236
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=25913
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=14792
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_tv.c

@@ -1153,6 +1153,15 @@ intel_tv_detect_type(struct intel_tv *intel_tv,
 		   DAC_B_0_7_V |
 		   DAC_C_0_7_V);
 
+
+	/*
+	 * The TV sense state should be cleared to zero on cantiga platform. Otherwise
+	 * the TV is misdetected. This is hardware requirement.
+	 */
+	if (IS_GM45(dev))
+		tv_dac &= ~(TVDAC_STATE_CHG_EN | TVDAC_A_SENSE_CTL |
+			    TVDAC_B_SENSE_CTL | TVDAC_C_SENSE_CTL);
+
 	I915_WRITE(TV_CTL, tv_ctl);
 	I915_WRITE(TV_DAC, tv_dac);
 	POSTING_READ(TV_DAC);