drm/i915/selftests: add test to verify get/put fw domains

Exercise acquiring and releasing forcewake around register reads. In
order to read a register behind a GT powerwell, we need to instruct that
powerwell to wake up using a forcewake. When we no longer require the GT
powerwell, we tell the GT to release our forcewake. Inside the
forcewake, the register read should work but outside it should just
return garbage, 0 being the most common garbage. Thus we can detect when
we are inside and outside of the forcewake with just a simple register
read, and so can verify that the GT powerwell is released when we say
so.

v2: Picking the right forcewaked register to return 0 outside of
forcewake is an art.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190320080052.27273-1-chris@chris-wilson.co.uk