drm/sun4i: dsi: Prevent underflow when computing packet sizes

Currently, the packet overhead is subtracted using unsigned arithmetic.
With a short sync pulse, this could underflow and wrap around to near
the maximal u16 value. Fix this by using signed subtraction. The call to
max() will correctly handle any negative numbers that are produced.

Apply the same fix to the other timings, even though those subtractions
are less likely to underflow.

Fixes: 133add5b ("drm/sun4i: Add Allwinner A31 MIPI-DSI controller support")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220812031623.34057-1-samuel@sholland.org