drm/i915/display: remove struct drm_i915_private forward declaration

Remove unused struct drm_i915_private forward declaration from
intel_display_core.h. Sort and group forward declarations while at it.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/fbccf45339a61711b377b35fd479a67b378c5571.1746529001.git.jani.nikula@intel.com


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