drm/panel: simple: Remove simple-panel compatible

simple-panel is not a valid panel model, so there is no data (video
timings, etc.) associated with it. Therefore drivers can't do anything
useful with it, so it should not appear in the table of OF matches.

Device trees will always need to specify the exact model of the panel.

Signed-off-by: Thierry Reding <treding@nvidia.com>