drm/i915/selftests: Measure dispatch latency

A useful metric of the system's health is how fast we can tell the GPU
to do various actions, so measure our latency.

v2: Refactor all the instruction building into emitters.
v3: Mark the error handling if not perfect, at least consistent.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200519130802.4067-1-chris@chris-wilson.co.uk