drm/omap: dsi: simplify VC handling

The VC handling has gotten quite tangled up. As the first step to clean
it up, lets define that we only support a single DSI peripheral (which
was really already the case), and we always use VC0 (define VC_DEFAULT
0) register block to send data to the peripheral.

We can thus have a single mipi_dsi_device pointer and remove the
for-loops which made passes over all the four VCs (just the first one
was ever used).

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/20201215104657.802264-63-tomi.valkeinen@ti.com