drm/nouveau/mxm: fix null deref on load

Since commit 61b365a505d6 ("drm/nouveau: populate master subdev pointer
only when fully constructed"), the nouveau_mxm(bios) call will return
NULL, since it's still being called from the constructor.  Instead, pass
the mxm pointer via the unused data field.

See https://bugs.freedesktop.org/show_bug.cgi?id=73791

Reported-by: Andreas Reis <andreas.reis@gmail.com>
Tested-by: Andreas Reis <andreas.reis@gmail.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Dave Airlie <airlied@linux.ie>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/nouveau/core/subdev/mxm/nv50.c

@@ -100,7 +100,7 @@ mxm_match_dcb(struct nouveau_mxm *mxm, u8 *data, void *info)
 static int
 mxm_dcb_sanitise_entry(struct nouveau_bios *bios, void *data, int idx, u16 pdcb)
 {
-	struct nouveau_mxm *mxm = nouveau_mxm(bios);
+	struct nouveau_mxm *mxm = data;
 	struct context ctx = { .outp = (u32 *)(bios->data + pdcb) };
 	u8 type, i2cidx, link, ver, len;
 	u8 *conn;
@@ -199,7 +199,7 @@ mxm_dcb_sanitise(struct nouveau_mxm *mxm)
 		return;
 	}
 
-	dcb_outp_foreach(bios, NULL, mxm_dcb_sanitise_entry);
+	dcb_outp_foreach(bios, mxm, mxm_dcb_sanitise_entry);
 	mxms_foreach(mxm, 0x01, mxm_show_unmatched, NULL);
 }