drm/i915: prefer i915_runtime_pm in intel_runtime function

As a first step towards updating the code to work on the runtime_pm
structure instead of i915, rework all the internals to use and pass
around that.

v2: add comment for kdev (Jani), move rpm init after pdev init for
mock_device

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613232156.34940-2-daniele.ceraolospurio@intel.com