drm/i915/guc: Remove function pointers for send/receive calls

Since we started using CT buffers on all gens, the function pointers can
only be set to either the _nop() or the _ct() functions. Since the
_nop() case applies to when the CT are disabled, we can just handle that
case in the _ct() functions and call them directly.

v2: keep intel_guc_send() and make the CT send/receive functions work on
    intel_guc_ct. (Michal)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217012316.13271-5-daniele.ceraolospurio@intel.com