drm/i915/selftests: Measure basic throughput of blit routines

We need to verify that our blitter routines perform as expected, so
measure it.

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/20191028112207.5464-1-chris@chris-wilson.co.uk