drm/panel: simple: Set appropriate mode type

All modes exposed by simple panels should be tagged as driver defined
modes. Moreover, if a panel supports only one mode, this mode is
obviously the preferred one.

Doing this also fix a problem occurring when a 'video=' parameter is
passed on the kernel command line. In some cases the user provided mode
will be preferred over the simple panel ones, which might result in
unpredictable behavior.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>
[treding@nvidia.com: reshuffle some code for consistency]
Signed-off-by: Thierry Reding <treding@nvidia.com>