drm/bridge: adv7533: Use internal timing generator

ADV7533 provides an internal timing generator for certain modes that it
can't use the DSI clock directly.

We've observed that HDMI is more stable with the internal timing
generator, especially if there are instabilities in the DSI clock source.
The data spec also seems to recommend the usage of the timing generator
for all modes.

However, on some platforms, it's reported that enabling the timing
generator causes instabilities with the HDMI output.

Create a DT parameter that lets a platform explicitly disable the timing
generator. The timing generator is enabled by default.

Signed-off-by: Archit Taneja <architt@codeaurora.org>