drm/i915: Fix kerneldoc parameter markup

drm/i915: Fix kerneldoc parameter markup

drivers/gpu/drm/i915/i915_syncmap.c:92: warning: No description found for parameter 'root'
drivers/gpu/drm/i915/i915_syncmap.c:155: warning: No description found for parameter 'root'
drivers/gpu/drm/i915/i915_syncmap.c:155: warning: No description found for parameter 'id'
drivers/gpu/drm/i915/i915_syncmap.c:155: warning: No description found for parameter 'seqno'
drivers/gpu/drm/i915/i915_syncmap.c:354: warning: No description found for parameter 'root'
drivers/gpu/drm/i915/i915_syncmap.c:354: warning: No description found for parameter 'id'
drivers/gpu/drm/i915/i915_syncmap.c:354: warning: No description found for parameter 'seqno'
drivers/gpu/drm/i915/i915_syncmap.c:396: warning: No description found for parameter 'root'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208105449.29880-2-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>