drm/i915/gvt: remove local storage of debugfs file

drm/i915/gvt: remove local storage of debugfs file

There is no need to keep the dentry around for the debugfs kvmgt cache
file, as we can just look it up when we want to remove it later on.
Simplify the structure by removing the dentry and relying on debugfs
to find the dentry to remove when we want to.

By doing this change, we remove the last in-kernel user that was storing
the result of debugfs_create_long(), so that api can be cleaned up.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: intel-gvt-dev@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210518161705.3697143-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>