drm/i915/selftests: Try to recycle context allocations

igt_ctx_exec allocates a new context for each iteration, keeping them
all allocated until the end. Instead, release the local ctx reference at
the end of each iteration, allowing ourselves to reap those if under
mempressure.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827161726.3640-2-chris@chris-wilson.co.uk