drm/i915/guc/ct: Drop guards in enable/disable calls

We track the status of the GuC much more closely now and we expect the
enable/disable functions to be correctly called only once. If this isn't
true we do want to flag it as a flow failure (via the BUG_ON in the ctch
functions) and not silently ignore the call.

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217012316.13271-2-daniele.ceraolospurio@intel.com