drm/mipi-dbi: Make mipi_dbi_command_stackbuf() data parameter const

mipi_dbi_command_stackbuf() copies the passed buffer data, so it can be
const.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316164249.6234-1-geert+renesas@glider.be