drm/sitronix: st7571-i2c: Select VIDEOMODE_HELPERS

This driver requires of_get_display_timing() from
CONFIG_VIDEOMODE_HELPERS but does not select it. If no other driver
selects it, there will be a failure from the linker if the driver is
built in or modpost if it is a module.

  ERROR: modpost: "of_get_display_timing" [drivers/gpu/drm/sitronix/st7571-i2c.ko] undefined!

Select CONFIG_VIDEOMODE_HELPERS to resolve the build failure.

Fixes: 4b35f0f4 ("drm/st7571-i2c: add support for Sitronix ST7571 LCD controller")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250610-drm-st7571-i2c-select-videomode-helpers-v1-1-d30b50ff6e64@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>