drm: zte: select VIDEOMODE_HELPERS in Kconfig

ZTE DRM driver uses drm_display_mode_to_videomode() in function
zx_crtc_enable().  Select VIDEOMODE_HELPERS in Kconfig to fix the
following link error.

  LD      vmlinux.o
  MODPOST vmlinux.o
drivers/built-in.o: In function `zx_crtc_enable':
:(.text+0xbdeb8): undefined reference to `drm_display_mode_to_videomode'

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>