drm/i915/guc: Make intel_guc_send a function pointer

Prepare for an alternate GuC communication interface.

v2: Make a few functions static and name them correctly while we are at it (Oscar), but
leave an intel_guc_send_mmio interface for users that require old-style communication.

v3: Send intel_uc_init_early back to the top (Michal).

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>