drm/sti/sti_hqvdp: Fix incorrectly named function 'sti_hqvdp_vtg_cb()'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/sti/sti_hqvdp.c:796: warning: expecting prototype for sti_vdp_vtg_cb(). Prototype was for sti_hqvdp_vtg_cb() instead

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Fabien Dessenne <fabien.dessenne@st.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602143300.2330146-9-lee.jones@linaro.org