drm/vc4: dsi: Use snprintf for the PHY clocks instead of an array

The DSI clocks setup function has been using an array to store the clock
name of either the DSI0 or DSI1 blocks, using the port ID to choose the
proper one.

Let's switch to an snprintf call to do the same thing and simplify the
array a bit.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201203132543.861591-4-maxime@cerno.tech