drm/i915/selftests: Skip using the GPU if wedged

If the GPU is irrecoverably broken, we can not use it to dirty memory
and check for cache coherency with the CPU. All we can do is simply skip
over the GPU subtests and focus on the CPU domains (WC, WB) cache
management.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107127
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706065332.15214-3-chris@chris-wilson.co.uk