drm/sun4i: dsi: Remove unused drv from driver context

This member is never used, so remove it.

Fixes: 133add5b ("drm/sun4i: Add Allwinner A31 MIPI-DSI controller support")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211072858.30784-1-samuel@sholland.org