drm/panel: sitronix-st7789v: simplify st7789v_spi_write

st7789v_spi_write initializes a message with just
a single transfer, spi_sync_transfer can be used
for that.

Reviewed-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714013756.1546769-7-sre@kernel.org