drm/i915: Reset only affected engines when handling error capture

Pass down the engine mask to i915_clear_error_registers so only affected
engines can be reset on the Gen6/7 path.

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/20190607082557.31670-1-tvrtko.ursulin@linux.intel.com