drm/nouveau: Ignore broken legacy I2C entries.

The nv05 card in the bug report [1] doesn't have usable I2C port
register offsets (they're all filled with zeros). Ignore them and use
the defaults.

[1] http://bugs.launchpad.net/bugs/569505

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bios.c

@@ -5167,10 +5167,14 @@ static int parse_bmp_structure(struct drm_device *dev, struct nvbios *bios, unsi
 	bios->legacy.i2c_indices.crt = bios->data[legacy_i2c_offset];
 	bios->legacy.i2c_indices.tv = bios->data[legacy_i2c_offset + 1];
 	bios->legacy.i2c_indices.panel = bios->data[legacy_i2c_offset + 2];
-	bios->dcb.i2c[0].write = bios->data[legacy_i2c_offset + 4];
-	bios->dcb.i2c[0].read = bios->data[legacy_i2c_offset + 5];
-	bios->dcb.i2c[1].write = bios->data[legacy_i2c_offset + 6];
-	bios->dcb.i2c[1].read = bios->data[legacy_i2c_offset + 7];
+	if (bios->data[legacy_i2c_offset + 4])
+		bios->dcb.i2c[0].write = bios->data[legacy_i2c_offset + 4];
+	if (bios->data[legacy_i2c_offset + 5])
+		bios->dcb.i2c[0].read = bios->data[legacy_i2c_offset + 5];
+	if (bios->data[legacy_i2c_offset + 6])
+		bios->dcb.i2c[1].write = bios->data[legacy_i2c_offset + 6];
+	if (bios->data[legacy_i2c_offset + 7])
+		bios->dcb.i2c[1].read = bios->data[legacy_i2c_offset + 7];
 
 	if (bmplength > 74) {
 		bios->fmaxvco = ROM32(bmp[67]);