drm/rockchip: add CONFIG_OF dependency

DRM_DISPLAY_DP_AUX_BUS cannot be selected when CONFIG_OF is disabled:

WARNING: unmet direct dependencies detected for DRM_DISPLAY_DP_AUX_BUS
  Depends on [n]: HAS_IOMEM [=y] && DRM [=y] && OF [=n]
  Selected by [y]:
  - DRM_ROCKCHIP [=y] && HAS_IOMEM [=y] && DRM [=y] && ROCKCHIP_IOMMU [=y] && ROCKCHIP_ANALOGIX_DP [=y]

Rockchip platforms all depend on OF anyway, so add the dependency here
for compile testing.

Fixes: d7b4936b ("drm/rockchip: analogix_dp: Add support to get panel from the DP AUX bus")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Acked-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250423164422.2793634-1-arnd@kernel.org