drm/sti/sti_hdmi_tx3g4c28phy: Provide function names for kernel-doc headers

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

 drivers/gpu/drm/sti/sti_hdmi_tx3g4c28phy.c:77: warning: expecting prototype for Start hdmi phy macro cell tx3g4c28(). Prototype was for sti_hdmi_tx3g4c28phy_start() instead
 drivers/gpu/drm/sti/sti_hdmi_tx3g4c28phy.c:187: warning: expecting prototype for Stop hdmi phy macro cell tx3g4c28(). Prototype was for sti_hdmi_tx3g4c28phy_stop() instead

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Vincent Abriou <vincent.abriou@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/20210602143300.2330146-6-lee.jones@linaro.org