drm/i915/gt: Fix memory leak in hangcheck selftest

drm/i915/gt: Fix memory leak in hangcheck selftest

In active_engines, if intel_context_create fails, we need to go
backwards through all the created contexts to free/put them.  However,
the way this is currently performed skips the first created context, as
if count == 1, then --count returns 0 and exits the while-loop
prematurely without performing the intel_context_put on context 0.

Fix this by post-decrementing count in the while-loop, rather than
pre-decrementing it.  This change makes the prior guard against count
underflowing unnecessary, as the while-loop exits when count == 0.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: Krzysztof Karas <krzysztof.karas@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20250904193041.12888-2-jonathan.cavitt@intel.com