drm/omap: fix maximum sizes

We define max width and height in mode_config to 2048. These maximums
affect many things, which are independent and depend on platform. We
need to do more fine grained checks in the code paths for each
component, and so the maximum values in mode_config should just be "big
enough" to cover all use cases.

Change the maximum width & height to 8192.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>