drm/i915: Use i915_hw_context to set reset stats

With full ppgtt support drm_i915_file_private gained knowledge
about the default context. Also reset stats are now inside
i915_hw_context so we can use proper abstraction.

v2: Move BUG_ON and WARN_ON to more proper locations (Ben)

v3: Pass dev directly to i915_context_is_banned to avoid the need to
dereference ctx->last_ring. Spotted by Mika when checking my
s/BUG/WARN/ change, I've missed this ->last_ring dereference.

Suggested-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com> (v2)
Reviewed-by: Ben Widawsky <ben@bwidawsk.net> (v2)
[danvet: s/BUG/WARN/]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>