drm/sti: use seq_puts to display a string

gdp_dbg_ctl() uses seq_printf() to display a color format name even
though there is no format string. When using -Wformat-string, gcc
reports the following warning:

    drivers/gpu/drm/sti/sti_gdp.c: In function 'gdp_dbg_ctl':
    drivers/gpu/drm/sti/sti_gdp.c:150:18: warning: format not a string
    literal and no format arguments [-Wformat-security]
        seq_printf(s, gdp_format_to_str[i].name);
                      ^~~~~~~~~~~~~~~~~

Silence this warning by using seq_puts() instead.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170331192507.20538-1-nicolas.iooss_linux@m4x.org