drm/nv50/disp: prevent false output detection on the original nv50

Commit ea9197cc323839ef3d5280c0453b2c622caa6bc7 effectively enabled the
use of an improved DAC detection code, but introduced a regression on
the original nv50 chipset, causing a ghost monitor to be detected.

v2 (Ben Skeggs): the offending line was likely a thinko, removed it for
all chipsets (tested nv50 and nve6 to cover entire range) and added
some additional debugging.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67382
Tested-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/engine/disp/dacnv50.c

@@ -49,18 +49,23 @@ int
 nv50_dac_sense(struct nv50_disp_priv *priv, int or, u32 loadval)
 {
 	const u32 doff = (or * 0x800);
-	int load = -EINVAL;
+
 	nv_mask(priv, 0x61a004 + doff, 0x807f0000, 0x80150000);
 	nv_wait(priv, 0x61a004 + doff, 0x80000000, 0x00000000);
+
 	nv_wr32(priv, 0x61a00c + doff, 0x00100000 | loadval);
 	mdelay(9);
 	udelay(500);
-	nv_wr32(priv, 0x61a00c + doff, 0x80000000);
-	load = (nv_rd32(priv, 0x61a00c + doff) & 0x38000000) >> 27;
-	nv_wr32(priv, 0x61a00c + doff, 0x00000000);
+	loadval = nv_mask(priv, 0x61a00c + doff, 0xffffffff, 0x00000000);
+
 	nv_mask(priv, 0x61a004 + doff, 0x807f0000, 0x80550000);
 	nv_wait(priv, 0x61a004 + doff, 0x80000000, 0x00000000);
-	return load;
+
+	nv_debug(priv, "DAC%d sense: 0x%08x\n", or, loadval);
+	if (!(loadval & 0x80000000))
+		return -ETIMEDOUT;
+
+	return (loadval & 0x38000000) >> 27;
 }
 
 int