drm/panel: Add and fill drm_panel type field

Add a type field to the drm_panel structure to report the panel type,
using DRM_MODE_CONNECTOR_* macros (the values that make sense are LVDS,
eDP, DSI and DPI). This will be used to initialise the corresponding
connector type.

Update all panel drivers accordingly. The panel-simple driver only
specifies the type for the known to be LVDS panels, while all other
panels are left as unknown and will be converted on a case-by-case
basis as they all need to be carefully reviewed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190904132804.29680-2-laurent.pinchart@ideasonboard.com