drm/i915: Make Gen6/7 RING_FAULT_REG access engine centric

Similar to earlier conversions, eliminate the implicit dev_priv by
introducing some helpers which take the engine parameter (since the
register itself is per engine).

v2:
 * Always use parentheses in macro arguments.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190607101535.767-1-tvrtko.ursulin@linux.intel.com