drm/amdgpu: use default_groups in kobj_type

There are currently 2 ways to create a set of sysfs files for a
kobj_type, through the default_attrs field, and the default_groups
field.  Move the amdgpu sysfs code to use default_groups field which has
been the preferred way since aa30f47c ("kobject: Add support for
default attribute groups to kobj_type") so that we can soon get rid of
the obsolete default_attrs field.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: John Clements <john.clements@amd.com>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Jonathan Kim <jonathan.kim@amd.com>
Cc: Kevin Wang <kevin1.wang@amd.com>
Cc: shaoyunl <shaoyun.liu@amd.com>
Cc: Tao Zhou <tao.zhou1@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>