tests/i915/gem_caching: handle discrete

Test should still be valid, even if we can't explicitly control the PTE
caching bits, like on discrete, where the caching should already be
enabled by default for system memory objects.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/4873


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@linux.intel.com>