Reproducible panic on "kldunload i915kms" as of r236822

I'm running CURRENT as of r236822 on my Thinkpad X220 and am starting
to experiment with the i915 kms driver. All packages are from the
pkgbeta repo; I haven't rebuilt anything yet.  I have
xorg-server-1.7.7_5,1 and the vesa driver gets used.  In this
configuration I have a reproducible panic unloading i915kms.ko.

Reproduction steps:

1. kldload i915kms && startx
2. switch back to the text console
3. ^C the x server
4. kldunload i915kms

Crash info is at http://people.freebsd.org/~emaste/i915kms_panic.txt -
interesting stack frames are:

#8  0xffffffff80ba22d3 in calltrap ()
   at /home/emaste/src/head-ro/sys/amd64/amd64/exception.S:228
#9  0xffffffff808e8213 in device_delete_child (dev=0xfffffe0007986a00,
   child=0xdeadc0dedeadc0de)
   at /home/emaste/src/head-ro/sys/kern/subr_bus.c:1859
#10 0xffffffff808e8224 in device_delete_child (dev=0xfffffe0007986b00,
   child=0xfffffe0007986a00)
   at /home/emaste/src/head-ro/sys/kern/subr_bus.c:1867
#11 0xffffffff81ad9b05 in iic_dp_aux_detach (idev=0xfffffe0007986b00)
   at /home/emaste/src/head-ro/sys/modules/drm2/drm2/../../../dev/drm2/drm_dp_iic_helper.c:230
#12 0xffffffff808e80f4 in device_detach (dev=0xfffffe0007986b00)
   at device_if.h:214
#13 0xffffffff808e8241 in device_delete_child (dev=0xfffffe0002727500,
   child=0xfffffe0007986b00)
   at /home/emaste/src/head-ro/sys/kern/subr_bus.c:1872
#14 0xffffffff81a8ddb6 in intel_dp_encoder_destroy (
   encoder=0xfffffe000721ca00)
   at /home/emaste/src/head-ro/sys/modules/drm2/i915kms/../../../dev/drm2/i915/intel_dp.c:2287
#15 0xffffffff81ad369f in drm_mode_config_cleanup (dev=0xfffffe0007bdc000)
   at /home/emaste/src/head-ro/sys/modules/drm2/drm2/../../../dev/drm2/drm_crtc.c:968
#16 0xffffffff81a64a82 in i915_driver_unload_int (dev=0xfffffe0007bdc000,
   locked=true)
   at /home/emaste/src/head-ro/sys/modules/drm2/i915kms/../../../dev/drm2/i915/i915_dma.c:1341
#17 0xffffffff81adb34a in drm_detach (kdev=Variable "kdev" is not available.
)
   at /home/emaste/src/head-ro/sys/modules/drm2/drm2/../../../dev/drm2/drm_drv.c:640

-Ed