drm/i915: Remove unused i915_gem_active_wait() in favour of _unlocked()

Since we only use the more generic unlocked variant, just rename it as
the normal i915_gem_active_wait(). The temporary cost is that we need to
always acquire the reference in a RCU safe manner, but the benefit is
that we will combine the common paths.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161028125858.23563-5-chris@chris-wilson.co.uk