drm: Merge __drm_atomic_helper_disable_all() into drm_atomic_helper_disable_all()

The only difference between the functions is the clean_old_fbs argument, whose
use was removed in the patch referenced below. So remove the internal
copy and drop the guts back into drm_atomic_helper_disable_all()

Changes in v2:
- Instead of just removing the unused arg, merge the functions

Fixes: e00fb856 ("drm: Stop updating plane->crtc/fb/old_fb on atomic drivers")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Sinclair Yeh <syeh@vmware.com> # This email bounces :(
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190212173245.51980-1-sean@poorly.run