drm/sti: Fix sparse warnings

drm/sti: Fix sparse warnings

drm/sti/sti_mixer.c:361:6: warning: symbol 'sti_mixer_set_matrix' was not declared. Should it be static?
drm/sti/sti_gdp.c:476:5: warning: symbol 'sti_gdp_field_cb' was not declared. Should it be static?
drm/sti/sti_gdp.c:885:24: warning: symbol 'sti_gdp_plane_helpers_funcs' was not declared. Should it be static?
drm/sti/sti_cursor.c:348:24: warning: symbol 'sti_cursor_plane_helpers_funcs' was not declared. Should it be static?
drm/sti/sti_compositor.c:28:28: warning: symbol 'stih407_compositor_data' was not declared. Should it be static?
drm/sti/sti_compositor.c:49:28: warning: symbol 'stih416_compositor_data' was not declared. Should it be static?
drm/sti/sti_vtg.c:75:1: warning: symbol 'vtg_lookup' was not declared. Should it be static?
drm/sti/sti_vtg.c:476:24: warning: symbol 'sti_vtg_driver' was not declared. Should it be static?
drm/sti/sti_dvo.c:109:5: warning: symbol 'dvo_awg_generate_code' was not declared. Should it be static?
drm/sti/sti_dvo.c:602:24: warning: symbol 'sti_dvo_driver' was not declared. Should it be static?
drm/sti/sti_vtac.c:209:24: warning: symbol 'sti_vtac_driver' was not declared. Should it be static?
drm/sti/sti_tvout.c:914:24: warning: symbol 'sti_tvout_driver' was not declared. Should it be static?
drm/sti/sti_hqvdp.c:786:5: warning: symbol 'sti_hqvdp_vtg_cb' was not declared. Should it be static?
drm/sti/sti_hqvdp.c:1253:24: warning: symbol 'sti_hqvdp_plane_helpers_funcs' was not declared. Should it be static?
drm/sti/sti_hqvdp.c:1292:5: warning: symbol 'sti_hqvdp_bind' was not declared. Should it be static?
drm/sti/sti_hqvdp.c:1385:24: warning: symbol 'sti_hqvdp_driver' was not declared. Should it be static?
drm/sti/sti_drv.c:143:6: warning: symbol 'sti_drm_dbg_cleanup' was not declared. Should it be static?

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>