drm/panel: add panel driver for Leadtek LTK500HD1829

The LTK500HD1829 is 5.5" DSI display.

v5:
  - Fix some trivial checkpatch warnings while applying (sam)

changes in v4:
- drop error message if backlight not found, no other panel
  does that and if needed it should live in drm_panel_of_backlight
changes in v3:
- drop one more overlooked panel->drm access

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/20191224112641.30647-3-heiko@sntech.de