drm/rockchip: fix typo in Kconfig 's/HDMI/dsi/'

In the help of ROCKCHIP_DW_MIPI_DSI it said
it is a HDMI driver instead of a dsi driver.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116141609.26719-1-dafna.hirschfeld@collabora.com