drm/i915/dmc: allocate dmc structure dynamically

sizeof(struct intel_dmc) > 1024 bytes, allocated on all platforms as
part of struct drm_i915_private, whether they have DMC or not.

Allocate struct intel_dmc dynamically, and hide all the dmc details
behind an opaque pointer in intel_dmc.c.

Care must be taken to take into account all cases: DMC not supported on
the platform, DMC supported but not initialized, and DMC initialized but
not loaded. For the second case, we need to move the wakeref out of
struct intel_dmc.

v2:
- Rebase to kzalloc dmc after runtime pm get (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/20230301122944.1298929-4-jani.nikula@intel.com