Unfortunately we've never really made use of creating/destroying connectors on the fly like we have with MST, so 4.6 ended up showing a lot of various bugs with hotplugging MST displays, booting with MST displays, etc. Most of these bugs are very likely to panic the kernel, and a couple of them end up even doing out of bounds memory accesses causing all sorts of other issues.
The proper fix for these issues is Dave's connector lifetime patch series in 4.7-rc1[1], however backporting those patches would be too big of a fix to submit for stable. This patch series is a much smaller set of changes to workaround this issue.
As another note: the first two patches in this series are already upstream for 4.7-rc1, however since we have the connector ref lifetime patches in 4.7-rc1 they don't fix any kernel panics there, only a few inconsistencies in i915/drm's code. They do however, fix kernel panics for 4.6 since connectors getting destroyed can make dev->mode_config.num_connector have a different value from fb_helper->connector_count.
[1]: 0552f7651bc233e5407ab06ba97a9d7c25e19580 in master
Lyude (4): drm/i915/fbdev: Fix num_connector references in intel_fb_initial_config() drm/fb_helper: Fix references to dev->mode_config.num_connector drm/i915: Discard previous atomic state on resume if connectors change drm/atomic: Verify connector->funcs != NULL when clearing states
drivers/gpu/drm/drm_atomic.c | 2 +- drivers/gpu/drm/drm_fb_helper.c | 5 ++--- drivers/gpu/drm/i915/intel_display.c | 12 ++++++++++++ drivers/gpu/drm/i915/intel_fbdev.c | 6 +++--- 4 files changed, 18 insertions(+), 7 deletions(-)