drm/i915/pmdemand: make struct intel_pmdemand_state opaque

drm/i915/pmdemand: make struct intel_pmdemand_state opaque

Only intel_pmdemand.c should look inside the struct
intel_pmdemand_state. Indeed, this is already the case. Finish the job
and make struct intel_pmdemand_state opaque, preventing any direct pokes
at the guts of it in the future.

Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/bc5f418785ecd51454761e9a55f21340470a92e3.1735662324.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>