drm/i915/guc: Splitting CT channel open/close functions

The aim of this patch is to allow enabling and disabling
of CTB without requiring the mutex lock.

v2: Phasing out ctch_is_enabled function and replacing it with
    ctch->enabled (Daniele)

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190220013927.9488-2-sujaritha.sundaresan@intel.com