drm/i915/dmc: add i915_to_dmc() and dmc->i915 and use them

Start preparing for dynamically allocated struct intel_dmc by adding
i915_to_dmc() and dmc->i915, and using them. Take the future NULL dmc
pointer into account already now, and add separate logging for
initialization in the DMC debugfs.

v3:
- Obtain runtime pm reference first (Imre)

v2:
- Don't reduce debugfs output (Imre)

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301122944.1298929-3-jani.nikula@intel.com