drm/i915: Fix comments mentioning typo in IS_ENABLED()

This has no code changes, but the typo is clearly getting copy/pasted,
so better to avoid this now and fix the typo. IS_ENABLED() takes full
names, and must have the "CONFIG_" prefix.

Reported-by: Joe Perches <joe@perches.com>
Link: https://lore.kernel.org/lkml/b08611018fdb6d88757c6008a5c02fa0e07b32fb.camel@perches.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/202006050718.9D4FCFC2E@keescook