sun6i: dsi: fix gcc-4.8

Older compilers warn about initializers with incorrect curly
braces:

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c: In function 'sun6i_dsi_encoder_enable':
drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c:720:8: error: missing braces around initializer [-Werror=missing-braces]
  union phy_configure_opts opts = { 0 };
        ^
drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c:720:8: error: (near initialization for 'opts.mipi_dphy') [-Werror=missing-braces]

Use the GNU empty initializer extension to avoid this.

Fixes: bb3b6fcb

 ("sun6i: dsi: Convert to generic phy handling")
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428215105.3928459-1-arnd@arndb.de