drm/vc4: kms: Convert to for_each_new_crtc_state

The vc4 atomic commit loop has an handrolled loop that is basically
identical to for_each_new_crtc_state, let's convert it to that helper.

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/a712d2b70aaee20379cfc52c2141aa2f6e2a9d5b.1599120059.git-series.maxime@cerno.tech