drm/i915: Include i915_reg.h in intel_ringbuffer.h

Header intel_ringbuffer.h is using definitions from i915_reg.h
but forget to include it. Remove this hidden dependency by
explicitly include missing header.

v2: add reminder (Chris)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180308095037.18264-2-michal.wajdeczko@intel.com