drm/vc4: crtc: Remove mode_set_nofb

On BCM2711 to avoid stale pixels getting stuck in intermediate FIFOs, the
pixelvalve needs to be setup each time there's a mode change or enable /
disable sequence.

Therefore, we can't really use mode_set_nofb anymore to configure it, but
we need to move it to atomic_enable.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f86c7a6946f98262f1cf59a461596a796d4bcc5f.1599120059.git-series.maxime@cerno.tech