drm: fix memory leak around mode_group (v2)

This mode group id_list was never being freed.

v2: take David's suggestion to free in minor_free.

Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/drm_crtc.c
Simple merge

diff --cc drivers/gpu/drm/drm_stub.c
Simple merge

diff --cc include/drm/drm_crtc.h
Simple merge