drm/panel: add panel driver for Leadtek LTK050H3146W

The LTK050H3146W is 5.0" 720x1280 DSI display. There are two variants
with essentially the same name, LTK050H3146W and LTK050H3146W-A2.

They differ in their init sequence and mode details.

changes in v2:
- add display variants
changes in v3:
- fixed indentation and artifacts found by Sam

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408085317.2624599-2-heiko@sntech.de