drm/i915/pm: use forward declaration to remove an include

The fewer includes the better.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dcb426e4c6497f9ee3356c5f4fd4aaabd6295262.1629281426.git.jani.nikula@intel.com