intel/gen_decoder: return -1 for unknown command formats

intel/gen_decoder: return -1 for unknown command formats

Decoding with aubinator encountered a command of 0xffffffff. With the
previous code, it caused aubinator to jump 255 + 2 dwords to start
decoding again.

Instead we can attempt to detect the known instruction formats. If the
format is not recognized, then we can advance just 1 dword.

v2:
 * Update aubinator_error_decode
 * Actually convert the length variable returned into a *signed* integer
   in aubinator.c, intel_batchbuffer.c and aubinator_error_decode.c.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>