drm/sti: Replace 17 seq_puts() calls by seq_putc()

Single characters should be put into a sequence at several places.
Thus use the corresponding function "seq_putc".

This issue was detected by using the Coccinelle software.

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/5b4e2964-0742-8367-976f-678356d9347a@users.sourceforge.net