drm/i915/debugfs: remove the i915_cache_sharing debugfs file

The i915_cache_sharing file is a debugfs interface for gen 6-7 with no
validation or user. Remove it.

This also removes the last I915_WRITE() use in i915_debugfs.c.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130111601.2817-4-jani.nikula@intel.com