drm/vc4: crtc: Use local chan variable

The vc4_crtc_handle_page_flip already has a local variable holding the
value of vc4_crtc->channel, so let's use it instead.

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/439c589baec72ddb89159857a2d078fdd77b02a2.1599120059.git-series.maxime@cerno.tech