drm/i915/selftests: mark up hugepages object with start_cpu_write

Just like we do for internal objects. Also just use
i915_gem_object_set_cache_coherency() here. No need for over-flushing on
LLC platforms.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018174508.2137279-9-matthew.auld@intel.com