drm/i915/selftests: Allow for larger engine counts

Increasing the engine count causes a couple of local array variables
to exceed the kernel stack limit. So make them dynamic allocations
instead.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721223043.834562-8-matthew.d.roper@intel.com