drm/i915/gt: Select the deepest available parking mode for rc6

On Ivybridge, we can go lower than rc6 to rc6p. And this is required for
Ivybridge to hit the same minimum power consumption as rc6 on other
platforms, so make it so.

v2: Update selftest to include all rc6 residency counters

Note that Andi did mention that we should be converting the magic
numbers into opaque magic macros, so if they ever get reused (unlikely
given only Ivybridge used the extra modes) we'll need to pay back the
technical debt.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1518
Fixes: 730eaeb5

 ("drm/i915/gt: Manual rc6 entry upon parking")
Testcase: igt/i915_pm_rc6_residency/rc6-idle
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200324134232.8773-1-chris@chris-wilson.co.uk