drm/sun4i: sun6i_mipi_dsi: fix DCS long write packet length

The packet length of DCS long write packet should not be added with 1
when constructing long write packet.

Fix this.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191006160303.24413-4-icenowy@aosc.io