drm/xe/guc: Refactor GuC load to use poll_timeout_us()

drm/xe/guc: Refactor GuC load to use poll_timeout_us()

Currently there are 2 wait loops for loading GuC: one in
xe_mmio_wait32_not() and one guc_wait_ucode(). Now that there's a
generic poll_timeout_us(), refactor the code to use that to be more
readable.

Main change in behavior is that there's no exponential wait anymore:
that is now replaced by a 10msec retry.

Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://lore.kernel.org/r/20250922-xe-iopoll-v4-5-06438311a63f@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>