drm/i915: re-init modeset hw state after gpu reset

After a gpu reset we need to re-init some of the hw state we only
initialize when modeset is enabled, like rc6, hw contexts or render/GT
core clock gating and workaround register settings.

Note that this patch has a small change in the resume code:
- rc6 on gen6+ is only restored for the modeset case (for more
  consistency with other callsites). This is no problem because recent
  kernels refuse to load drm/i915 without kms on gen6+
- rc6/emon on ilk is only restored for the modeset case. This is no
  problem because rc6 is disabled by default on ilk, and ums on ilk
  has never really been a supported option outside of horrible rhel
  backports.

v2: Chris Wilson noticed that we not only fail to restore the clock
gating settings after gpu reset.

v3: Move the call to modeset_init_hw in _reset out of the
struct_mutext protected area - other callers don't hold it, too.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>