drm/nouveau: fix panels using straps-based mode detection

nouveau_bios_fp_mode() zeroes the mode struct before filling in relevant
entries.  This nukes the mode id initialised by drm_mode_create(), and
causes warnings from idr when we try to remove the mode.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>