drm/i915/gsc: Disable GSC engine and power well if FW is not selected

The GSC CS is only used for communicating with the GSC FW, so no need to
initialize it if we're not going to use the FW. If we're not using
neither the engine nor the microcontoller, then we can also disable the
power well.

IMPORTANT: lack of GSC FW breaks media C6 due to opposing requirements
between CS setup and forcewake idleness. See in-code comment for detail.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: John C Harrison <John.C.Harrison@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208200521.2928378-6-daniele.ceraolospurio@intel.com