drm/bridge: cdns-dsi: Select VIDEOMODE_HELPERS

drm/bridge: cdns-dsi: Select VIDEOMODE_HELPERS

When no other driver selects CONFIG_VIDEOMODE_HELPERS but
CONFIG_DRM_CDNS_DSI is enabled, there is a linker or modpost error:

  ERROR: modpost: "drm_display_mode_to_videomode" [drivers/gpu/drm/bridge/cadence/cdns-dsi.ko] undefined!

Select VIDEOMODE_HELPERS to ensure that this helper function is
available to the driver.

Fixes: ce4bc5ca7c1d ("drm/bridge: cdns-dsi: Use video mode and clean up cdns_dsi_mode2cfg()")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250821-cdns-videohelpers-v1-1-853e021908cf@kernel.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>