drm/i915/display: drop unused declarations from intel_display.h

We've accumulated lots of forward declarations in intel_display.h that
are no longer necessary. Clean them up.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/5ad046b74040e84fab51786c346ff9a445e351bc.1747128495.git.jani.nikula@intel.com


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