drm/i915/selftests: Constrain mock_gtt tests to fit within RAM

Be pessimistic and presume that we actually allocate every page we
exercise via the mock_gtt (e.g. for gvt). In which case we have to keep
our working set under the available physical memory to prevent oom.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180710080424.7821-1-chris@chris-wilson.co.uk