intel: batch-decoder: don't asks for constant BO until decoding

intel: batch-decoder: don't asks for constant BO until decoding

With PPGTT mappings, our aubinator implementation can be quite slow if
we request a buffer that doesn't exist. Instead of doing a PPGTT walk
for invalid addresses (0 lengths), wait until we're sure we want to
decode the data.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>