drm/i915/selftests: fix the uint*_t types that have crept in

Always prefer the kernel types over stdint types in i915.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210113141317.30765-1-jani.nikula@intel.com