drm/i915: Make i915_spin_request() static

drm/i915: Make i915_spin_request() static

No users now outside of i915_wait_request(), so we can make it private to
i915_gem_request.c, and assume the caller knows the seqno. In the
process, also remove i915_gem_request_started() as that was only ever
used by i915_spin_request().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170922120333.25535-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>