udlfb: Add module_param to allow forcing pixel_limit

Some user scenarios need to prioritize performance over
maxiumum resolution.

Also, some devices may have bad vendor descriptors, and
this allows the user to set a pixel limit that matches
their specific device to avoid blank screens on higher
resolution monitors.

700000 minimum for DL-115, 2360000 maximum for DL-195

Signed-off-by: Ben Collins <bcollins@ubuntu.com>
Signed-off-by: Bernie Thompson <bernie@plugable.com>