drm/tinydrm/mipi-dbi: Select DRM_KMS_HELPER

mipi-dbi uses several KMS helper functions but that build dependency is
not expressed. Select DRM_KMS_HELPER to fix that.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722104312.16184-8-noralf@tronnes.org