drm/i915/guc: rename loader entry points

The GuC initialisation code could do other things apart from loading
firmware, so here we rename the three primary entry points to remove any
specific reference to "ucode" (no functional changes, just renaming).

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>