drm/sti: awg_utils code cleanup

data_en is a local variable that doesn't need to be set as
awg_generate_instr can be called directly with the requested value.

Signed-off-by: Bich Hemon <bich.hemon@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>