drm/i915: remove dangling forward declaration

Commit 20a7f2fc ("drm/i915: Convert intel_mocs_init_l3cc_table to
intel_gt") removed the only user.
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730182614.14379-1-lucas.demarchi@intel.com

drivers/gpu/drm/i915/gt/intel_mocs.h

@@ -49,7 +49,6 @@
  * context handling keep the MOCS in step.
  */
 
-struct drm_i915_private;
 struct i915_request;
 struct intel_engine_cs;
 struct intel_gt;