drm/i915/guc: Extract GuC stage desc pool creation into a helper

drm/i915/guc: Extract GuC stage desc pool creation into a helper

Since it's a two-step process, we can have a cleaner error handling in
the caller if we do the allocations in a helper.

Signed-off-by: Michał Winiarski <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Jeff McGee <[email protected]>
Cc: Michal Wajdeczko <[email protected]>
Cc: Oscar Mateo <[email protected]>
Reviewed-by: Daniele Ceraolo Spurio <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]