drm/tegra: Clarify how panel modes override others

When a panel advertises one or more modes, they are used exclusively.
Other methods for obtaining the mode, such as DDC as used for HDMI or
binary EDID blobs embedded in the DT, are ignored. The panel drivers
should be providing this functionality if they want to expose it as
well.

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