drm/sti: Fix typos in a comment line

Adjust this description for a function call.

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/e70bf37b-a461-873e-4ba3-40bf43cc7b21@users.sourceforge.net