video: tl070wsh30: Fix use of CONFIG_IS_ENABLED on DM_REGULATOR

The DM_REGULATOR symbol has SPL counterpart in:
  drivers/power/regulator/Kconfig:config SPL_DM_REGULATOR
Use CONFIG_IS_ENABLED() macro to match on the correct
variant depending on the build stage.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>