drm/gud: rearrange gud_probe() to prepare for function splitting

drm/gud: rearrange gud_probe() to prepare for function splitting

gud_probe() is currently very large and does many things, including
pipeline setup and feature detection, as well as having USB functions.

This patch re-orders the code in gud_probe() to make it more organised
and easier to split apart in the future.

Signed-off-by: Ruben Wauters <rubenru09@aol.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20251020140147.5017-1-rubenru09@aol.com/