drm/i915: Rename i915_gem_timelines_mark_idle

The kerneldoc markup for i915_gem_timelines_mark_idle() was incorrect,
so take the opportunity to also convert it from the "mark_idle" to "park"
naming scheme.

drivers/gpu/drm/i915/i915_gem_timeline.c:120: warning: No description found for parameter 'i915'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171127123054.20966-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>