drm/i915: Verify the engine workarounds stick on application

Read the engine workarounds back using the GPU after loading the initial
context state to verify that we are setting them correctly, and bail if
it fails.

v2: Break out the verification into its own loop

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190417075657.19456-3-chris@chris-wilson.co.uk