drm/nv17-nv4x: Fix analog load detection false positive on rare occasions.

On some boards the residual current DAC outputs can draw when they're
disconnected can be high enough to give a false load detection
positive (I've only seen it in the S-video luma output of some cards,
but just to be sure). The output line capacitance is limited and
sampling twice should fix it reliably.
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv04_dac.c

@@ -291,6 +291,8 @@ uint32_t nv17_dac_sample_load(struct drm_encoder *encoder)
 	msleep(5);
 
 	sample = NVReadRAMDAC(dev, 0, NV_PRAMDAC_TEST_CONTROL + regoffset);
+	/* do it again just in case it's a residual current */
+	sample &= NVReadRAMDAC(dev, 0, NV_PRAMDAC_TEST_CONTROL + regoffset);
 
 	temp = NVReadRAMDAC(dev, head, NV_PRAMDAC_TEST_CONTROL);
 	NVWriteRAMDAC(dev, head, NV_PRAMDAC_TEST_CONTROL,