drm/i915/selftests: Add tests for GT and engine workaround verification

Two simple selftests which test that both GT and engine workarounds are
not lost after either a full GPU reset, or after the per-engine ones.

(Including checks that one engine reset is not affecting workarounds not
belonging to itself.)

v2:
 * Rebase for series refactoring.
 * Add spinner for actual engine reset!
 * Add idle reset test as well. (Chris Wilson)
 * Share existing global_reset_lock. (Chris Wilson)

v3:
 * intel_engine_verify_workarounds can be static.
 * API rename. (Chris Wilson)
 * Move global reset lock out of the loop. (Chris Wilson)

v4:
 * Add missing rpm puts. (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/20181203125014.3219-5-tvrtko.ursulin@linux.intel.com