drm/i915/selftests: use prandom in selftest

This is part of a prandom cleanup, which removes
next_pseudo_random32 and replaces it with the standard PRNG.

Signed-off-by: Markus Theil <theil.markus@gmail.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250211063332.16542-2-theil.markus@gmail.com