drm/sti/sti_hda: Provide missing function names

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

 drivers/gpu/drm/sti/sti_hda.c:283: warning: expecting prototype for Search for a video mode in the supported modes table(). Prototype was for hda_get_mode_idx() instead
 drivers/gpu/drm/sti/sti_hda.c:301: warning: expecting prototype for Enable the HD DACS(). Prototype was for hda_enable_hd_dacs() instead
 drivers/gpu/drm/sti/sti_hda.c:383: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
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/20210602143300.2330146-7-lee.jones@linaro.org