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