drm: silence variable 'conn' set but not used

drm: silence variable 'conn' set but not used

The "struct drm_connector" iteration cursor from
"for_each_new_connector_in_state" is never used in atomic_remove_fb()
which generates a compilation warning,

drivers/gpu/drm/drm_framebuffer.c: In function 'atomic_remove_fb':
drivers/gpu/drm/drm_framebuffer.c:838:24: warning: variable 'conn' set
but not used [-Wunused-but-set-variable]

Silence it by marking "conn" __maybe_unused.

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1563822886-13570-1-git-send-email-cai@lca.pw