drm/i915: Verify power domains after enabling them

After
commit 2cd9a689

 ("drm/i915: Refactor intel_display_set_init_power() logic")
it makes more sense to check the power domain/well refcounts after
enabling the power domains functionality. Before that it's guaranteed
that most power wells (in the INIT domain) will have a reference held,
so not an interesting state.

While at it also add the check after the init_hw/fini_hw, disable and
suspend/resume steps. Make the test optional on a Kconfig option since
it may add substantial overhead: on VLV/CHV the corresponding PUNIT reg
access for each power well may take up to 20ms.

v2:
- Add the state check to more spots. (Chris)

v3:
- During suspend check the state before deiniting display core.
  Afterwards DC states are disabled (and so the dc_off power well is
  enabled) even though we don't hold a reference on it.
- Do the test conditionally based on a new Kconfig option. (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[Add DRM_I915_DEBUG_RUNTIME_PM to welcome messages]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180817145837.26592-1-imre.deak@intel.com