New test pm_sseu is intended for any subtest related to the
slice/subslice/EU power gating feature. The sole initial subtest,
'full-enable', confirms that the slice/subslice/EU state is at
full enablement when the render engine is active. Starting with
Gen9 SKL, the render power gating feature can leave SSEU in a
partially enabled state upon resumption of render work unless
explicit action is taken.

v2: Add test description and apply recommendations of igt.cocci
    (Thomas Wood).
v3: Skip instead of fail if debugfs entry i915_sseu_status is not
    available.

Signed-off-by: Jeff McGee <jeff.mcgee@intel.com>
Tested-by: Lei Liu <lei.a.liu@intel.com>
Signed-off-by: Thomas Wood <thomas.wood@intel.com>