drm/sti: do not post HQVDP command if no update

Do not process update requests with unmodified parameters.

Since the HQVDP command queue is limited to 2, we shall take care of
not posting unneeded commands, which would abusively fill the command
queue leading to frame update skip.
This typically happens when the driver is called with legacy
(non-atomic) IOCTL : in that case atomic_update() is called multiple
times with the same parameters.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>