panel-sitronix-st7701: Fix panel prepare over SPI

A DSI write is issued in st7701_prepare even when the probed panel
runs on SPI. In practice, this results in a panic with the
vc4-kms-dpi-hyperpixel2r overlay active.

Perform the equivalent write over SPI in this case.

Signed-off-by: Jack Andersen <jackoalan@gmail.com>