video: tegra: refuse to bind to disabled dcs

This prevents the following boot-time message on any board where only the
first DC is in use, yet the DC's DT node is enabled:

stdio_add_devices: Video device failed (ret=-22)

(This happens on at least Harmony, Ventana, and likely any other Tegra20
board with display enabled other than Seaboard).

The Tegra DC's DT node represents a display controller. It may itself
drive an integrated RGB display output, or be used by some other display
controller such as HDMI. For this reason the DC node itself is not
enabled/disabled in DT; the DC itself is considered a shared resource, not
the final (board-specific) display output. The node should instantiate a
display output driver only if the rgb subnode is enabled. Other output
drivers are free to use the DC if they are enabled and their DT node
references the DC's DT node. Adapt the Tegra display drivers' bind()
routine to only bind to the DC's DT node if the RGB subnode is enabled.

Now that the display driver does the right thing, remove the workaround
for this issue from Seaboard's DT file.

Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>