drm/i915/gt/uc: Evaluate GuC priority within locks

The ce->guc_state.lock was made to protect guc_prio, which
indicates the GuC priority level.

But at the begnning of the function we perform some sanity check
of guc_prio outside its protected section. Move them within the
locked region.

Use this occasion to expand the if statement to make it clearer.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240613222402.551625-2-andi.shyti@linux.intel.com