drm/sti: Fix a typo in a comment line

Add a missing character in this description for a data structure.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/a5a1f6cb-ba9d-0f7d-9126-9aeffb3783ad@users.sourceforge.net