modetest: get the crtc_id from the pipe_arg

Makes the code shorter and easier to read.

Currently if the user has not set the crtc_id, we fetch the crtc yet do
not "bother" setting the id - do so.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Ezequiel Garcia <ezequiel@collabora.com>