drm/i915/pmdemand: convert to_intel_pmdemand_state() to a function

In preparation for making struct intel_pmdemand_state an opaque type,
convert to_intel_pmdemand_state() to a function.

Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/10324781f9f7eae5a92506aaa7a40403efd345dd.1735662324.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>