drm/panel-sitronix-st7703: Drop custom DSI write macros

There are macros for these already in the <drm/drm_mipi_dsi.h> header, use
that instead and delete the custom DSI write macros defined in the driver.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230107191822.3787147-3-javierm@redhat.com