drm: panel: Fix bpc for OrtusTech COM43H4M85ULC panel

The OrtusTech COM43H4M85ULC panel is a 18-bit RGB panel. Commit
f098f168 ("drm: panel: Fix bus format for OrtusTech COM43H4M85ULC
panel") has fixed the bus formats, but forgot to address the bpc value.
Set it to 6.

Fixes: f098f168 ("drm: panel: Fix bus format for OrtusTech COM43H4M85ULC panel")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200824003254.21904-1-laurent.pinchart@ideasonboard.com