drm/panel: jh057n00900: Don't use magic constant

0xBF isn't in any ST7703 data sheet so mark it as unknown. This avoids
confusion on whether there is a missing command in that
dsi_generic_write_seq() call.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/7f50fa6de2ae029111f158f8ea7fd69a0903eb97.1561542477.git.agx@sigxcpu.org