drm/i915: drop a number of unnecessary forward declarations

Remove leftovers from earlier cleanups.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230118131538.3558599-3-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -65,13 +65,6 @@
 #include "intel_uncore.h"
 
 struct drm_i915_clock_gating_funcs;
-struct drm_i915_gem_object;
-struct drm_i915_private;
-struct intel_connector;
-struct intel_dp;
-struct intel_encoder;
-struct intel_limit;
-struct intel_overlay_error_state;
 struct vlv_s0ix_state;
 
 #define I915_GEM_GPU_DOMAINS \