drm/i915/bdw: Do gen6 style reset for gen8

This patch existed before, but was lost over time.

Note that reset is still somewhat problematic in my limited testing (ie.
module_reload will not pass) but it can be disabled with a module
parameter, and support should be considered preliminary anyway.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>