drm/tinydrm/mipi-dbi: Move cmdlock mutex init

cmdlock protects command execution so put it in mipi_dbi_spi_init()
where it conceptually belongs.
This is prep work for the splitting of struct mipi_dbi.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722104312.16184-2-noralf@tronnes.org