drm/i915/guc : GEM_BUG_ON on invoking GuC reset function

Instead of returning -EINVAL, GEM_BUG_ON when GuC reset is invoked for
platforms not supporting as we don't expect to invoke it.

v2: re-wording commit message and subject (Sagar)

Signed-off-by: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1514928025-29659-2-git-send-email-sujaritha.sundaresan@intel.com