Inside i915_gem_reset(), we start touching the HW and so require the
low-level HW to be re-enabled, in particular the PCI BARs.

Fixes: 7b6da818 ("drm/i915: Restore the kernel context after a GPU reset on an idle engine")
References: 0db8c961 ("drm/i915: Re-enable GTT following a device reset")
Testcase: igt/drv_hangman #i915g/i915gm
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171217132852.30642-1-chris@chris-wilson.co.ukReviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>