drm/tinydrm/mipi-dbi: Add drm_to_mipi_dbi()

Add a function to derive mipi_dbi from drm_device now that tinydrm_device
is going away.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190210131039.52664-8-noralf@tronnes.org