drm/sun4i: dsi: fix the overhead of the horizontal front porch

The formula in the BSP kernel indicates that a 16-byte overhead is used
when sending the HFP. However, this value is currently set to 6 in the
sun6i_mipi_dsi driver, which makes some panels flashing.

Fix this overhead value.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191006160303.24413-3-icenowy@aosc.io