drm/omap: dsi: rename 'channel' to 'vc'

The "channel" usage in omap dsi driver is confusing. We have three
different "channels":

1) DSI virtual channel ID. This is a number from 0 to 3, included in the
packet payload.

2) VC. This is a register block in the DSI IP. There are four of those
blocks. A VC is a DSI "pipeline", with defined fifo settings, data
source (cpu or dispc), and some other settings. It has no relation to
the 1).

3) dispc channel. It's the "pipeline" number dispc uses to send pixel
data.

The previous patch handled the third case.

 To start fixing 1) and 2), we first rename all uses of 'channel' to
'vc', as in most of the cases that is the correct thing to use.

However, in some places 1) and 2) have gotten mixed up (i.e. the code
uses msg->channel when it should use vc), which will be fixed in the
following patch.

Note that mixing 1) and 2) currently is "fine", as at the moment we only
support DSI peripherals with DSI virtual channel 0, and we always use
VC0 to send data. So both 1) and 2) are always 0.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-66-tomi.valkeinen@ti.com