configs: stm32: Enable CONFIG_DM_REGULATOR for stm32f769-disco

Since commit 5bc6f8c2("video: stm32: remove test on CONFIG_DM_REGULATOR")
backlight was broken with the following message at boot:
stm32-display-dsi dsi@40016c00: Warning: cannot get phy dsi supply
stm32_display display-controller@40016800: panel panel enable backlight error -38

DM_REGULATOR flag must be enabled to fix this issue

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>