drm/i915: Refactor intel_display_set_init_power() logic

The device global init_power_on flag is somewhat arbitrary and makes
debugging power refcounting problems difficult. Instead arrange things
so that all display power domain get has a corresponding put call. After
this change we have the following sequences:

driver loading:
intel_power_domains_init_hw();
<other init steps>
intel_power_domains_enable();

driver unloading:
intel_power_domains_disable();
<other uninit steps>
intel_power_domains_fini_hw();

system suspend:
intel_power_domains_disable();
<other suspend steps>
intel_power_domains_suspend();

system resume:
intel_power_domains_resume();
<other resume steps>
intel_power_domains_enable();

at other times while the driver is loaded:
intel_display_power_get();
...
intel_display_power_put();

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180816123757.3286-2-imre.deak@intel.com