drm/i915: Use ABI engine class in error state ecode

Instead of printing out the internal engine mask, which can change between
kernel versions making it difficult to map to actual engines, present a
bitmask of hanging engines ABI classes. For example:

  [drm] GPU HANG: ecode 9:8:24dffffd, in gem_exec_schedu [1334]

Engine ABI class is useful to quickly categorize render vs media etc hangs
in bug reports. Considering virtual engine even more so than the current
scheme.

v2:
 * Do not re-order fields. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105113842.1395391-1-tvrtko.ursulin@linux.intel.com