drm/i915 Fix typos in i915_gem_fence.c

This patch fix some spelling typos found in Documentation/Docbook
gpu/ch04s03.html.  This file was generated from comments within
source, so I have to fix typos in i915_gem_fence.c.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>