drm/i915: Reset the gpu on takeover

The GPU may be in an unknown state following resume and module load. The
previous occupant may have left contexts loaded, or other dangerous
state, which can cause an immediate GPU hang for us. The only save
course of action is to reset the GPU prior to using it - similarly to
how we reset the GPU prior to unload (before a second user may be
affected by our leftover state).

We need to reset the GPU very early in our load/resume sequence so that
any stale HW pointers are revoked prior to any resource allocations we
make (that may conflict).

A reset should only be a couple of milliseconds on a slow device, a cost
we should easily be able to absorb into our initialisation times.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170124110135.6418-2-chris@chris-wilson.co.uk