drm/i915/guc: prefer 'dev_priv' to 'dev' for static functions

Convert all static functions in i915_guc_submission.c that currently
take a 'dev' pointer to take 'dev_priv' instead (there are three,
guc_client_alloc(), guc_client_free(), and gem_allocate_guc_obj().

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>