drm/i915: Initialise basic fence before acquiring seqno

drm/i915: Initialise basic fence before acquiring seqno

Inside the intel_timeline_get_seqno(), we currently track the retirement
of the old cachelines by listening to the new request. This requires
that the new request is ready to be used and so requires a minimum bit
of initialisation prior to getting the new seqno.

Fixes: a114d350b3b3 ("drm/i915: Coordinate i915_active with its own mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203094152.4150550-2-chris@chris-wilson.co.uk
(cherry picked from commit 855e39e65cfc33a73724f1cc644ffc5754864a20)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>