drm/i915: Verify GT workaround state after GPU init

Since we now have all the GT workarounds in a table, by adding a simple
shared helper function we can now verify that their values are still
applied after some interesting events in the lifetime of the driver.

Initially we only do this after GPU initialization.

v2:
 Chris Wilson:
 * Simplify verification by realizing it's a simple xor and and.
 * Remove verification from engine reset path.
 * Return bool straight away from the verify API.

v3:
 * API rename. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203125014.3219-4-tvrtko.ursulin@linux.intel.com