intel: decoder: unify MI_BB_START field naming

The batch decoder looks for a field with a particular name to decide
whether an MI_BB_START leads into a second batch buffer level. Because
the names are different between Gen7.5/8 and the newer generation we
fail that test and keep on reading (invalid) instructions.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107544


Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit f430a37f)