drm: sun4i: fix probe error handling

gcc points out a possible uninitialized variable use in
sun4i_dclk_create():

drivers/gpu/drm/sun4i/sun4i_dotclock.c: In function 'sun4i_dclk_create':
drivers/gpu/drm/sun4i/sun4i_dotclock.c:139:12: error: 'clk_name' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  init.name = clk_name;

The warning only shows up when CONFIG_OF is disabled, and the
property is never filled, but the same bug can show up even
when CONFIG_OF is enabled but of_property_read_string_index
returns another error.

To fix it, this ensures that sun4i_dclk_create propagates
any error from of_property_read_string_index.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9026e0d1 ("drm: Add Allwinner A10 Display Engine support")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>