drm/i915/guc: Don't GEM_BUG_ON on corrupted G2H CTB

We should never BUG_ON on any corruption in CTB descriptor as
data there can be also modified by the GuC. Instead we can
use flag "is_in_error" to indicate that we will not process
any further messages over this CTB (until reset). While here
move descriptor error reporting to the function that actually
touches that descriptor.

Note that unexpected content of the specific CT messages, that
still complies with generic CT message format, shall not trigger
disabling whole CTB, as that might just indicate new unsupported
message types.

v2: drop redundant message (Daniele)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@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/20200117082039.65644-2-michal.wajdeczko@intel.com