drm: Introduce __setplane_atomic()

All the plane->fb/old_fb/crtc dance of __setplane_internal() is
pointless on atomic drivers. So let's just introduce a simpler
version that skips all that.

Ideally we could also skip the __setplane_check() as
drm_atomic_plane_check() already checks for everything, but the
legacy cursor/"async" .update_plane() tricks bypass that so
we still need to call __setplane_check(). Toss in a FIXME to
remind someone to clean this up later.

v2: Use drm_drv_uses_atomic_modeset() (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705185907.9524-1-ville.syrjala@linux.intel.com