drm/i915/selftests: Perform some basic cycle counting of MI ops

Some basic information that is useful to know, such as how many cycles
is a MI_NOOP.

v2: Keep volatile pages pinned at all times! (Matthew)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Anna Karas <anna.karas@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111172716.23733-1-chris@chris-wilson.co.uk