drm/vc4: crtc: Rename HVS channel to output

In vc5, the HVS has 6 outputs and 3 FIFOs (or channels), with
pixelvalves each being assigned to a given output, but each output can
then be muxed to feed from multiple FIFOs.

Since vc4 had that entirely static, both were probably equivalent, but
since that changes, let's rename hvs_channel to hvs_output in the
vc4_crtc_data, since a pixelvalve is really connected to an output, and
not to a FIFO.

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/b7618bb17b1c435c5d6ce50bcde2fe9243281d02.1599120059.git-series.maxime@cerno.tech