drm/i915/display: drop a number of dependencies on i915_drv.h

With the switch to an unordered workqueue dedicated to display, we've
stopped using struct drm_i915_private in a number of places, and can
drop the dependencies on i915_drv.h.

Cc: Luca Coelho <luciano.coelho@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20250626101636.1896365-1-jani.nikula@intel.com